It may well be that some application or process is "hogging" the machine cycles - waiting for a response from somewhere perhaps - a response that never comes.
You can use Task Manager to see where the CPU time is being used up - you might also consider using System restore to go back to a point in time when it was working fine.
Consider also the entries in the Startup list - all sorts of rubbish gets itself an entry in there.
Peccavi
December 2010