I currently have Windows Vista. I tried upgrading to Windows 7 with the software I purchased, but it won't let me. What could be the problem?

I ran the Windows 7 Upgrade Adviser, and it said everything is compatible——that I can upgrade to a 32-bit Windows 7 from my Windows Vista. But when I tried installing yesterday night, it said “could not upgrade” and that my old Windows Vista would be restored. Should I try installing again? Anyone else had this problem before?