I know this is a PITA, but I have to recommend a clean install.
It's a ton more work, but there are real benefits. You system will be clean as a whistle and there won't be a ton of unnecessary files and drivers lying around to slow things down and potentially cause problems down the road. No matter what path you take, BACK UP YOUR FILES FIRST. If things go badly during the upgrade, you may need them.
I'm running Win7 64 bit at home and work, and love it. It's a solid, fast, stable operating system. It's worth it.
My
- N.F.H.