OS X :: Windows Installation Finished With Error - Cannot Restart Mac

Every time I try to turn on my computer it immediately goes to the grey screen with the different languages that says you have to hold down the power button until it restarts b/c there's some error. Here's the story behind it. I tried to install windows but Bootcamp wouldn't work. It said I had to back up my computer and reinstall the OS as Mac OS Extended (Journaled) (even though it already was!). But I didn't have an external HD that was big enough. I borrowed a friend's external HD and reinstalled from scratch (without putting my old system on yet).

The Windows installation finished, then said there was an error and to restart. It kept saying that and wouldn't restart. Then I tried going back to my Mac OS, and it immediately went to the grey screen with the different languages that says you have to hold down the power button until it restarts b/c there's some error. I even tried reinstalling my old system and it still did this. The disk repair utility said everything on the Mac OS should be fine, though. Eventually I was able to reinstall it clean from the Snow Leopard DVD, but now I can't get my old computer back or windows on it!