There really isn't an easy way.

If you install Windows over any other linux operating system you will lose
the boot loader. Windows takes over, in other words, it will be impossible
to start anything but Windows. There are ways to repartition and keep both
but the linux boot loader will need repair and this is not easy.

It is far better to install linux over Windows, which always allows dual
booting, i. e. you can choose Windows or Linux at startup. But, since you
have passed the point of doing that I'll have to let more expert users here
walk you through the process of adding Windows to Linux.

You could simply try to install your CAD/CAM software in Nux using Wine.
Or you can install a virual machine software ans install windows in that.

true, you could do that... i havent had much luck with virtual machines with
anything besides linux as an vm on a windows machine. But i have had some
luck with wine, great program

Since I have tried this before, my advice will simply be use a back up program
(with Linux there are ones that actually create a bootable iso) then format
install windows then reinstall your backup. ALOT less of a headache!

You may be able to use your software using Windows installed into
Virtual Box but it would be a good idea to first verify whether that
software is compatible with running on a virtual machine. What's the
name of the program ?

Are you opposed to buying a new drive and putting windows on that? You
would have more space for your projects and would not have to resize and try
a risky windows over linux install. I assume this is your lively-hood and
it's important to you, it would be a few extra bucks well spent. I have had
problems in the past with losing access to the windows boot loader and not
being able to access Windows at all. The result was spending many hours
learning about the linux boot process. Time well spent, but you don't want
to have to do it with a deadline over your head. Since then, I have kept
bare metal windows installations on their own separate drive, and if I can't
reach Windows through GRUB, I can just pull the drive out, set the Windows
drive as master and boot straight into it. It's inevitable if you are
learning linux that you will make some mistake that can jeopardize your
system, everyone here has made them. It's best to limit the possible
damage, especially if your income depends on it :)

Let me ask a dumb question.

I bought a Dell laptop on Ebay with Ubuntu pre-installed. It was an old machine
and is awkward to work with now, but I really liked the experience. How easy is
it to get an all-Linux machine and the heck with Windows? Why pay for a software
which I don't really need?

Now let me really expose my ignorance: Is there a way of getting a machine
without an OS and installing Linux on it? Why pay for someone else to install a
free program?

Several ways to get a PC without an OS on it really. Build yourself
from scratch / bare-bones systems which have most parts in place, just
need to add in a HD / some hardware vendors supply ready built full
systems without OS's too.

Just as a FYI - most vendors don't pay anything like the price we
would have to for Windows. They use volume licencing deals which cut
the price of Windows to almost nominal compared to the rest of the
parts and cost of assembly / testing / shipping. So really you aren't
paying that much of a premium for Windows and erasing it isn't a loss
of a great deal of money - just a case of whether even that small
amount going to MicroSoft's coffers rankles or not

I agree about the boot loader problem, but it can be re-written from the
live disk. His larger problem is protecting his data and reorganizing his
drive.
I would backup in any case. The best thing would be as LAC suggests. Install
Windows first. Windows likes to be number one and it ignores other OSes and
file systems. It will even want to format over your ext file system. For
that reason it is best to do your partitioning from Linux which identifies
other file systems, unless you know what you are doing and are confident.
An experienced Linux user could keep Ubuntu and install Windows last. It is
not that hard. However a newbie might prefer to backup and start fresh.
There are additional advantages perhaps not thought about when you first
installed such as setting up a separate home partition.

Adding a new drive would mean not having to do anything to the existing
one. And I agree, backup in any case. You just have to be comfortable
removing drives and setting the jumpers. The steps would be:

(shutdown, unplug)
1: Pull out existing HDD
2: Install new HDD
3: Insert windows install disk, power up, proceed with install
(shutdown)
4: Put linux drive in master position with jumper set to master
5: Put windows drive in slave position with jumper set to slave
5: Boot into linux and update grub

I think it's pretty tidy, and easy enough for a beginner. I'm sure you can
do this without removing the drives, but I'm not too familiar with the
windows install process.

As far as rewriting the boot loader, like I said, I wouldn't want to have to
figure that one out as a newbie under a deadline. This way he can swap the
drive positions and *know *he get back to work in minutes, and mess with the
boot loader later. Not that it's that difficult, but it's a big added
stress to have to reinstall a boot loader you know nothing about when you
have real work to do. And this kind of thing always happens at the wrong
time, for me anyway.

I also agree that setting up a separate /home partition for your linux
system is a good idea, this will be easy to do if you decide to reinstall
your linux system. Somewhat harder from an existing system, but there are
step by step tutorials in the forums.

IMHO, unplugging a desktop computer before working on it is a bad idea. With
the computer turned off, the only live voltage floating around inside the case
is the low-current power switch sense line. When you unplug the computer, you
are using your ground, and the computer is more sensitive to static buildup. I
also recommend wearing an anti-static wrist strap grounded to the computer
chassis whenever working inside the box.

Most IDE hard drives are now operated with the drive jumpers set for cable
select, so master/slave status is determined by where the drive sits on the IDE
cable. I don't know about the jumper settings on SATA drives.

I for one, always go in and change the jumpers to specify which drive I
want as the master and slave on an IDE drive. Sata drives do not use
jumpers as you can only connect one to each sata port.

Not a good idea to leave the power cable plugged in and switched on
whilst working inside a system. That low standby voltage isn't a
danger to the user but it sure is bad news for components such as
memory sticks and PCI/AGP/PCIe cards. Removing / fitting any parts
with voltage still present ( even residual voltage after unplugging )
can lead to damage - memory sticks being particularly vulnerable.

Anti-static straps are good as far as they go - but the danger is that
the crocodile clip comes adrift unnoticed and you think you are safe
when you are not. Personally I prefer to work bare-armed and keep in
contact with the case - it also helps working on a wooden floor and
also not wearing synthetic fibre clothing.

The proper thing to do when working inside the case it unplug your
system from electric and the discharge the remaining voltage by pressing
and holding your power button for 5 seconds.