Have you read the link in Allan's post? It will depend on your version.
http://applehelpwriter.com/2011/09/21/how-to-uninstall-mackeeper-malware/
This user tip explains a bit, why we are concerned: Do not install MacKeeper
I see, your primary reason to have it installed it, is to spead up your your Mac, but by automatically cleaning caches, etc, it may slow down your system even more. The caches are built, to speed up the system, so it will not have to compute the data over and over again. E.g. one of the files it removes (com.apple.xpchelper.cache) can make your mac slow as a snail, when playing videos.
If you have been running MacKeeper, reinstall your MacOS X - you can use the Combo Updater:
OS X Mountain Lion Update v10.8.4 (Combo)
If your mac is really unexpectedly slow, check your hardware and software, and the way you are using your mac - some ideas are here:
12 Reasons Why Your Mac Runs Slow With Mountain Lion | ChrisWrites.com