All I can suggest
by MarkFlax - 3/13/12 4:42 AM
other than a complete new install, is to first scan the Vista system with a couple of anti-malware scanners like the free versions of Malwarebytes Anti-malware and SUPERAntiSpyware.
See if they turn up anything.
Is anything mentioned in the Event Viewer? I wouldn't have thought so on an upgrade, but there might be a clue.
Also, there may be logs about the upgrade that could indicate why the upgrade failed. The Microsoft link below explains a similar, but not identical, situation where upgrading fails;
See if you have logs of the upgrade.