Do a virus scan ASAP! I'm not saying that is the cause but there is a nasty IE vulnerability found this week and the bad guys are being quick to exploit it.
http://www.pcworld.com/businesscente...erability.html
Also check your event viewer it should contain any errors relating to crashing or non responsive applications. Event viewer can be found via CONTROL PANEL->ADMINISTRATIVE TOOLS->EVENT VIEWER.
Good luck.
Keith.