Do You Have A Compaq/HP Desktop With...
an AMD processor? If you do, the Microsoft SP3 will hang your computer just as you say. HP has a fix for that at:
http://h10025.www1.hp.com/ewfrf/wc/genericSoftwareDownloadIndex?cc=us&dlc=en&lc=en&softwareitem=pv-60484-2&jumpid=reg_R1002_USEN
Hope this helps you.
Scott