I'll explain exactly what I did, perhaps that could help you.
I installed Windows 7 RC (x64) on one partition (taking up the entire drive), then it started giving me issues. I shrank Windows 7's partition to roughly half, then installed Windows Vista HP SP2 (x64) on the empty partition.
Then I deleted the Windows 7 partition, and then it wouldn't let me 'extend' with the unallocated space that I recieved from deleting Windows 7.
Someone mentioned that if the unallocated space is on the 'left' side, it has issues. Not sure if thats true or not, seems a bit odd if it is true.
It didn't ask me such a thing. I want to extend the unallocated space into C: so its all one partition.