Several knowledgeable members of this forum are using MSE, and are happy with (and recommend) it.

[If you are looking for a free program, there aren't many other good choices... avast is a very good free alternative, but it's more complicated to use ---- it has many "options" that users can "tweak", if they're interested].

I would NOT run Ad-Aware with MSE...  First off, I believe Ad-Aware currently includes an anti-virus component, and you DON'T want to be running two real-time anti-virus programs.   Secondly, even if you can disable Ad-Aware's anti-virus component, keeping only "the rest" of the features, I believe there are currently much better anti-malware programs available.

Should I run a scan from a different antivirus supplier occassionally? Microsoft recommends only running their Security Essentials and no other antivirus software.

After you uninstall anti-virus software, it is always a good idea to run a follow-up with the vendor's removal tool for your version of software just to be sure remnants that might interfere with your new AV have been removed.

If you are looking for free programs, please see these pages for:

LISTS OF FREE SECURITY SOFTWARE  

FREE SECURITY SOFTWARE

I concur with both BB and ky.

I use MSE on 2 Win 7 systems, with absolutely no problems, after 6 months. I love its simplicity. It has probably the lowest incidence of false positive detections of any AV I've used. It has no problem co-existing with my (rather large) stable of software defenses and on-demand scanners.

I felt this was a safe move to make based on the increased security afforded by Win 7 & IE9 compared to their predecessors on my older sytems.

As far as a "second opinion" online AV scan, I agree with BB's recommrndations - but only if you are having problems. Not only do these online scans take a fair amount of time, but the chances of detections being false positives are increased when you have no issues.

For routine "2nd opinion" AV scans, you might consider Hitman Pro, a free online scanner that does not conflict with your real-time AV. I have it installed on all my systems, and it has never given me any problems.  It utilizes engines from 5 reputable AVs,  scans only targeted areas of your system, and only takes a few minutes to run.
http://www.surfright.nl/en/home/

I use MSE essentials on four systems running Windows 7. It has a very simple interface and updates daily.  I have been using MSE for over two years without any problems.  I have several other systems that came with McAfee Security Center subscriptions installed, and I have an occasional problem or two with McAfee.

I run MSE on all my personal systems. In my opinion, one of the best commercial AVs out there is Kaspersky followed by Webroot.  

I run MSE on my Windows 7 system -- never had a problem with it on that OS. As far as commercial (paid) AV's, I prefer ESET's products.

Free - Microsoft Security Essentials

Paid - Kaspersky / Nod32 (Esset)

I suggest you to run in with another antivirus program. Microsoft Security Essentials can't protect your computer when you only have it.

I suggest you to run in with another antivirus program. Microsoft Security Essentials can't protect your computer when you only have it.

MSE just won the 2011 Reader's Choice LifeHacker Award for Best Security Suite.  (Suite???)

http://www.lifehacker.com.au/2011/12/best-security-suite-lifehacker-award-winners/

Note:   I myself use avast, but [as stated above] believe those who use MSE are making a good choice.   However, as a believer in layered protection, I do recommend that ANY stand-alone anti-virus be supplemented by the likes of MBAM PRO, WOT [set to block], and a HOSTS file.   I find OpenDNS's Family Shield and SpywareBlaster offer good additional protection, and believe that sandboxing is prudent.