A new version is out: a-squared 4.5.0.21: http://www.emsisoft.com/en/software/free/
Quote:

Version 4.5.0.21 - 10/7/2009 - for Windows XP, 2003/2008 Server, Vista and 7, full functionality on x64, 55 MB

Source: http://www.emsisoft.com/en/software/download/

Supports even Windows 7 and all 64-bit OSes! Rumor has it that the number of false positives are under control now and has been for a while. We'll just have to hope it's true.

I started using it again about a month or so ago after noticing they use Ikarus antivirus in it. It only produces two false positives on my system which are easily ignored via the settings.

The only reason I started using it again are because it will scan deep into compressed files which MBAM doesn't do, and since it's like a second on-demand antivirus.

I downloaded this ages ago and wasn't impressed,much FPs..so uninstalled.
Think twice to start use again.If don't mind,let me know how it goes..performance,detection etc.

thank

Well, I actually trust the reports about a significant reduction in the numbers of FP. Mods, Admins and other staff members on the best and biggest boards/forums seems to agree on this. EmsiSoft really did get their act together, one learns from reliable sources.

Performance:
* It's virtually invisible even if letting the service run in the background, I personally have it set to manual that way the service starts when the main app does.
* The scans can be slow, for instance to perform a Deep Scan ("complete/full system scan") it takes almost one full hour on my system.
* Updating it needs to be done several times each day, "otherwise the updates will be huge if skipping several days" - that's what I read about it on a forum however since I update a few times each day I haven't noticed anything too big.

Detection:
I really don't know how that stacks up against its rivals since it has nothing to find on my system and only finds two rather ancient files that it detects but those are known false positives. From what I've noticed it is seemingly missing the same stuff that Avast misses. However MBAM fills the gap in those misses.

My main use for it is to scan downloaded files via right click, and since it's able to scan inside setup files, etc., I'll keep it installed forever.

I downloaded from EmsiSoft but found it was not the new version. I Don't know if it is just something weird that happened to me but I then tried the download from Softpedia and it was the new version (4.5.0.21).
Download via Softpedia

Don't forget that FileHippo also hosts the download here, which is where I got it from.

I decided to disable the service (it's a slower machine and the difference is noticeable), and haven't noticed any difference. Is there any reason one shouldn't do that?

Quoting myself! If anyone's interested this is how it appears to work: Disabling the service will kill the program after running it. Having it set to manual appears to leave it running until restarting the computer.