I think the best anti-virus you can use is NONE. Just switch to Linux and forget about all of that.
Of course, if you must use Windows, then I would tell you to use either Avast or AVG, since I've used them and never had any problems. I believe I used Avira once and it wasn't very effective, but that was many moons ago...
As for the Ctrl+Alt+Del question, it is hard-coded into the OS, and AFAIK there's no way to disable it. Something screwy must be going on, like maybe a rogue app is catching that combo before it gets through to the OS layer. If this is the case, open up the task manager and look at the "processes" tab and see if you can find anything wrong there.
Hope that helps!