I would consider using Wubi in your situation. It installs in Windows and does
not alter your Windows configuration at all. It is not as good as installing it
on a partition of its own, but it isn't bad. There are advantages. You can
install and de-install it easily. It is no threat to your data or Windows since
it is on your drive as one big file. You can choose which drive to install it to
and need to know nothing about partitioning.

The disadvantage of Wubi is that it will slow if your Windows drive becomes
fragmented since it resides on the Windows files system. However, if you are
careful not to let it become fragmented then it will work just fine. Other than
not being able to hibernate, Ubuntu with Wubi works the same as the real thing.
The key is to set your file size large enough to give yourself room to grow.

If you elect for the full installation, I would consider consolidating data from
the D and G drives onto one or the other (probably D) and freeing up one drive
(probably G) for Ubuntu. Just be aware that Ubuntu won't use the C:, D: and G:
designation. You will know the drive by its size though. It will likely be
called hda1 for C and hda2 for D and hdb for G or something similar. Also it
will tell you the current file format. If you use the FAT 32 (G) then you will
know you have the right drive for sure. Allow Ubuntu to use the full drive if
you go with G and you should be fine.

I installed 8.04 on a external HD on my main computer. It was Drive F
according to XP, and installed as a dual boot with no problems at all.