Maybe Windows doesn't like one of your parts. Download the LiveCD of Ubuntu (or another free OS) and try installing that and if you still get an error, at least you'll know it's not Windows being dumb.
Edit: Also, did you have to skip any steps in your install? (Like they didn't want to install) When I installed Windows, I had to skip a few things, but it still worked. If you skipped something critical to running Windows, then the install may have completed, but it still might not work.