|System errors can sometimes cause this.. you'll find this a lot in Windows Vista as well.. a pain really. |
One thing I did for my laptop that had this same issue was a system restore. Go into Start>Programs>Accessories>System Tools>System Restore. There will be a calendar there after you select the option to restore to an earlier time, select a bolded date on the calendar where you know for a fact it was working, then restore it. You might have to try more than once for it to work, but select different dates if you have to do it multiple times.