Hi, if this has only just started happening have you made any changes to hardware or software recently, if so that might give a clue as to why.
Or, have you tried a Restore to see if that makes any difference. Or, just the old fav's of doing some disk maintenance i.e. DiskCleanup, running antivirus and ad-aware etc, followed by clear out of temp Internet files & cookies, followed by a defrag. Any or all of these could make a difference.