[SOLVED] What is the best free anti-virus for Windows 7?

SpybotSD creates host entries and browser entries to protect you from adware/malware sites so NO it is not useless to just run it periodically. Another program you might consider running periodically is Malwarebytes. See our recommended freeware programs.

Having two active anti-virus programs is never a good idea because of the performance issues, which definitely seems to be an issue with you.

MSE is the only active anti-virus recommended by this forum as a whole. Some of us use other active AV programs without issues but when your system is slow or getting BSODs it is often recommended to remove your AV to see if it is, in fact, detrimental to your system. So yes, I would suggest removing ad-aware and seeing if your system performance improves.

I'll MSE has so far proved to be best free AV for Windows7

There are many "free" AV/Malware products on the market. MSE, Avira, Avast, AVG are among the more popular ones. With all of the bad press on it, I'd stay away from AVG, it has (on this very forum) been shown, and proven, to create BSOD's.

I'd recommend going with one of the first three that I listed above, and use Malwarebytes (MBAM) to scan behind it. No security app (free or paid) is 100% right, all of the time. There was also a well known security suite that had problems recently on their site.

I just prefer to have more than one product on hand, and suggest to others to do the same, no matter which solution you choose. MBAM is effective, and free.

And finally, no matter which you pick, the best one(s) that you choose are only as good as how you use them. Keep it/them updated, run regular scans, avoid clicking onto unwanted ads and "spam" emails. If something looks too good to be true, it usually is.

Cat