At some point I had heard that disabling aero on windows 7 64x via registry would free up more RAM than doing it via preferences.
Needless to say I followed steps to remove aero via registry tweaking and now I'm trying to get it back. I have no idea what site i was at when i found this worthless information so i can't back track my steps or what not.
Basically when i go to preferences, the option for the aero theme is grey and un-clickable so doing this had managed to make it impossible to ever get aero again in any conventional method.
Don't know why you just didn't uncheck "Enable Transparency" when you right click on the desktop and go to personalize...that is disabling aero. If you have a system restore before you did that might be your only chance. I don't feel like googling how to solve it.
Edit : If you don't have a restore point, you may be stuck with re installing windows or doing a repair, to see if that helps. Lesson learned I guess?