There's a known problem with Norton / Symantec products and both Vista and XP - are you using Norton / Symantec?
Norton Product Protection (Symantec Resource Protection) causes the problem - see the Symantec website for details / workarounds.
If it's not that, I can only suggest a search for
"Your computer cannot be restored" "no changes have been made"
on the MS site or Google.
SD