I've said before thatI usually leave my computer on for extended periods and have set up my anti-virus to update at 4am and scan at 5am, no problems.
You should be able to install the free version of the AVG (www.grisoft.com) and turn off the auto tests, and it has a shell extension that allows you to right-click and scan files or directories. You probably want to leave the updates turned on though.
Dave