I finally got it to work, half via Command Prompt and half by pure frustration/multiple reboots. And yet I still have no idea why it all of a sudden decided to let me create new partitions again.
For the record, never try using the CLEAN ALL command unless you want to spend a long time waiting for it to finish cleaning. The CLEAN command will do the trick if, like me, you're just trying to clean your disk so you can install Windows on it again.
For anyone who's wondering, check out the post by gregrocker: W7 Installation problem - "Windows cannot be installed to Disk 0...
Seems to have done the trick for me. I still have no idea why Windows wasn't letting me peruse the options in the usual way that I do when formatting. Aaaand that's the half-day wasted mark. Thanks Microsoft!