Did your desktop come with vista preinstalled? If so then you can use the recovery program to wipe and reformat the drive or you can use the installation CD to also do it.
If you use the recovery method:
Once the disc has been reformated then vista reinstalled you can use the built in utillity to resize the the drive and partition it according to the program which is a bit less than half in most cases.

If you use the CD installation disc then you can set up the drive to what ever partition sizes you want!