Two quick thoughts that may help.
Have you tried this shutdown & restart troubleshooter?
http://aumha.org/win5/a/shtdwnxp.htm
If nothing there works are you able to download HijackThis from here ....
http://www.majorgeeks.com/download3155.html
....to either the infected computer or to a CD/floppy another one?
If so then scan the infected PC with it and post the final Notepad log report to this thread. It may reveal what your problem is.