Have a little used PC which is dual boot (on separate HDs) Win7 and XP. Going to use it the other day after some time and Win7 (and other things) decided to update - perfectly normal. Let it carry on. After re-booting it stuck on 'configuring your computer' then reset. And went through this cycle constantly.
XP works normally.
I've done an HD check and it seems ok.
Tried to do a system restore - but it says no restore points which is plain wrong.
Tried the original installation disc - my own Win7, not a OEM one - and it says it can't repair it. But nothing unusual there.