Windows 7 can start causing problems before it's even installed, as many people report their upgrade hangs forever at 62%. Which is annoying.
Reboot, and your PC should roll back to Windows Vista.
You can then open the setup log file \$WINDOWS.~BT\Sources\Panther\setupact.log to view what happened. Microsoft say this is usually caused because the Iphlpsvc service has stopped responding, and just adding an environment variable to ignore it will fix the problem.
Point your browser at Upgrade stops responding (hangs) at 62% when you upgrade to Windows 7
for the fix.
If this doesn't help (or your upgrade hangs at something other than 62%) then browse the setup log for other clues. And you might also try to boot and install from the Windows 7 disc, if possible, as that reduces the chance of any conflict with your existing Vista (or XP) setup.