Did you try uninstalling and then reinstalling the driver ?
As to why , don't know , did you format and upgrade or just upgrade ?
It's always a bad idea to upgrade the existing OS , you will get problems like yours.
Its a better plan to boot of the Upgrade CD it will detect a windows installation , but ask it to format and then upgrade , you may need the original XP cd , i never have , once it sees the original OS on the HD it shoud be happy to continue...

What you then have is a "clean" upgrade , if you get what im saying !!!!!