you can try this:
run -> msconfig -> startup -> take a look if threre is programs that you dont recognize...if there is disable those (note that those names don't necessarily be the same as in normal use...looka at the "command" and there you can see the path for that aplication). Disable all unknow aplications (make sure you dont disable firewall or display drivers)....reboot and see if that helps....if not do it again and disable all but firewall and those drivers.
If this solves the problem....go back to msconfig and one by one turn on those programs ...reboot after every singe program and try ctrl+alt+del until you find the program that disables it...uninstal it if it's a program you have installed yourself and if it's not paste the program path here and we can find out what it is.