I have similar problem and have also contacted Microsoft but the advice doesn't help at all:
"We believe that you have install a windows vista service pack 2 beta
version which have caused this issue. To be able to resolve this case,
please uninstall the Windows Vista Service Pack 2. For guided steps to
uninstall it, refer to the link below: http://support.microsoft.com/kb/948537"
I never had SP2 beta installed.
Wingmaker, if you get more constructive advice please post it here.