|All anti-malware software slows down Windows. Some is known to slow it down a lot more than others, and the slower you computer is in the first place, the more you'll notice the difference.|
Microsoft Security Essentials slows down your computer less than most anti-malware software, but if it's like the situation with Microsoft's Windows Defender, it can't run in Safe mode, and if so, I wouldn't use it.
Sometimes you need to run anti-malware software in Safe mode, and most anti-malware software can do that.
(Windows Defender also can't be un-installed in Safe mode.)
The free version of AVG 2011 slows down Windows about the same amount as Microsoft Security Essentials, a little less than Avast! does, and it can run in Safe mode.
No one anti-malware program can protect you against all malware these days in any case.
As a second anti-malware program, I recommend you download and install the free version of Malwarebytes. It finds and gets rid of a lot of malware other anti-malware programs don't find. Run it when running a full scan of your other anti-malware software doesn't help and you stilI suspect you have malware. It does not slow down your computer at all unless you're running it, and even then it doesn't slow it down much. It has no resident module that runs all the time so it can't clash with the resident modules of other anti-malware software.
However, the free version does not update itself after it is initially installed. If you need to run it after the day it was installed, click on the Update tab in the program,update it,THEN run a Full scan.
It can run and be un-installed in Safe mode.