You can repartion your hard drive(s) at any time using software such as Partition Magic. Unfortunately such software is a little expensive, running around $70. With the standard tools in Windows you can create partitions, but only from unpartitioned space, meaning you have to do it before you set everything up unless you're willing to start over now.
Hope this helps,
I have Win XP. On my 70 gig hard drive, I still have over 40gig free. Or is that something you can only do from the git-go.