here is what I do 99% of the time.

Download Malwarebytes
reboot computer and hit F9 - then boot in safe mode (if you can't download first, go in safe mode with networking and install malwarebytes there)
start -> run -> type in msconfig
go to startup tab, disable all.
reboot computer and let it go in to windows like normal. (Windows will start the necessary services, so disabling all doesn't affect anything)
then run full scan on malwarebytes
computer should be in a normal state to give you time to find some good virus protection software.

Goodluck!