All of a sudden my Dimension with Windows XP started saying it was downloading an update and then restarted itsself several times.  It has a Windows Installer window that never seems to finish or go away.  I tried to get McAfee security thinking I might have a virus but the Windows installer won't let it run to install.   THe Windows installer just cranks along but never seems to finish.

Any ideas on what might be wrong or how to fix? 

Thanks!!