Recently I began to run out of space on my Windows partition, mostly because when I installed Vista I thought that 30GB for the primary partition would be enough.
That proved to be a miscalculation, even disabling everything I could, like Hibernation and making Service Pack 1 permanent, I was still running out of space.
Luckily I found out that you can Merge partitions on Windows Vista, with no effort at all!
I had an old partition which I mostly used to test Linux distros, so I deleted that partition and merged up the free space with the primary partition, the problem was solved with a few clicks!
If you are running out of space, instead of reinstalling everything from scratch, you can backup a secondary partition, delete it, merge the free space with the Windows partition and live happily ever after.