Windows, since Vista, has included rather aggressive monitoring of any "unapproved" programs that you attempt to install or run. There was a fair amount of variation in just how aggressive the blockade was with Vista, depending on your security settings. With Win7 you probably get more warnings, although they've made it a little easier to "give permission," and you can install and run almost anything you choose to.

I have a couple of programs that I migrated to Win7, that I've used since ca. 2002, but since the most useful one doesn't have a current "certificate" Win 7 demanded that I "approve" installation as an exception, demands that I "approve" every opening of the program, and (it's a graphics editor) "approve" each and every file that I open in the program. (This makes it quite tedious since for what I use it most for, I may need to open as many as 100 or so files before I can start working them.) Some relief is obtained by setting the shortcut to open the program to "run as administrator" but it still balks and staggers. There is an updated program, but it's been "stupidicated" (similar to recent Office versions) to the point where the new version really is unusable. (Newer Office being merely painful and disgusting.)

The Microsoft AV program is free, and according to reviews is comparable to or better than the better free AV programs you can get elsewhere. Assuming that you get other Microsoft updates on schedule (i.e. assuming you're not a total ...) you probably can assume that it will be more reliably - and promptly - kept up to date than several others.

(An AV with updates a month old is sort of like having no AV.)

You generally should not run more than ONE AV program at a time. Having others on the machine, for "spot checks" when something suspicious occurs, is probably okay, if you're careful about which ones are running and when. Several of the programs mentioned here are much more suited (e.g. especially free MalwareBytes**) for detecting and removing infections already on your computer, and may not be particularly good at blocking incoming potential infections.

** In the most recent comparative tests I've seen, free Malwarebytes detected about as many currently common infections as most others, but was unable to remove, or failed to remove completely, almost a third of the ones it found. It has, according to older reports, been able to remove a couple of "stubborn" ones that other more sophisticated programs (for a while) had difficulty with, but overall performance has been sort of mediocre, and it provides no blocking to prevent infections. The paid version gets generally good ratings.

John