I have a desktop computer. My W7 drive has two partitions, System (C:), OS
(D:) where System is W7 and OS is HP's recovery partition. I used to have
it partitioned with the addition of several more partitions. In Linux the
drive was sda with sda1 being C: and sda2 being D: then I had and sda3
which was formatted to FAT 32 for shared data such as music and backups.
and then I had an extended partition for the remainder. Inside that I
created three partitions for Kubuntu, sda4 (20 GBs) for / (root), sda5
(4GBs) for SWAP and sda6 (200 GBs) for /home.

I said "used to have" because the drive started to fail and I added a
second HD (sdb) where I now have three partitions for Kubuntu 12.04 plus
nine other partitions for other distros (12 partitions in all) plus the two
on the original drive. I have had more in the past.

Partitioning need not be painful once you understand the basics.

1. Shrink the partition (in your case it will be C:) keeping it the size
that works for you. Keep the freed space to the right. That will keep C: in
place. Windows does not like you to mess with it.

2. Create one or more partitions in the freed or unallocated space using
the file system of your choice. For Linux I prefer three partitions --- a
separate root and home partition plus swap. In your case you can get by
with two (no home) and that way you do not need to use an extended
partition. What you would lose is a separate home which is very handy, but
not necessary. Alternatively you could create an extended partition and
partition that to create three or more partitions. It is just an extra
step, but going with two partitions requires less planning and may save
space. No separate home is just more work if you need to re-install.

Possible configurations for you could be

sda1 (C:), sda2 (D:) and sda3 (/) plus sda4 (swap) Your sda3 would be ext4
file system while sda1 and sda would show as NTFS for C: and probably FAT32
for D:

or

sda1 (C:), sda2 (D:) and sda3 being the extended partition with sda4 (/),
sda5 (/home) plus sda6 (swap). sda4 and sda5 would be ext4.

The size of sda3 and sda4 are dependent on your habits and hardware. If you
do not install many applications or store much data then you can get away
with 8 GBs. Your swap should not exceed your RAM (some say twice your RAM,
but more is not better since disk caching actually is slower than using
RAM). If you do lots of multimedia or plan on installing many applications
then you will need to increase to a higher partition size. Ripping a single
DVD can eat up several GBs in temp files alone. My basic configuration
would be a minimum of 20 GBs for Kubuntu, more if you have bigger HD.

Once you have partitioned then you can begin installing. Use the
partitioning software that you fell most comfortable with. If you lack any
Windows ones then you can use GParted or Parttion Manager that comes with
Kubuntu. I prefer to install from a usb key and use Unetbootin to create
the bootable ISO, but a CD works well. It is just much slower.

Check every decision twice before you commit it whether you are
partitioning or installing. Basically you will format only
one partition when installing. That will be your root partition (two if you
are using a separate home). Make sure that you are not formatting sda1 or
sda2 (your Windows partitions) whatever you do. Everything else is
recoverable, but if you format W7 then you are skewered. BTW, if you have
not burned recovery CDs for W7 then now would be a good time. Murphy's Law
suggests that if anything goes wrong it will and it will happen when you
are least prepared for it. An easy way to tell drives apart if you get
confused by letters is the file format. Windows will show as NTFS and FAT.
Don't touch anything in those formats.