MacKeeper is highly invasive malware* that can de-stablize your operating system. It is also unethically marketed and a rip-off.
Further opinion on it and how to uninstall MacKeeper malware:
http://applehelpwriter.com/2011/09/21/how-to-uninstall-mackeeper-malware/
Do NOT download or use the ‘MacKeeper uninstaller’ from the Zeobit site, as this will cause even more damage to your operating system.
This is also worth reading:
http://www.reedcorner.net/news.php/?p=245
Equally phoney is iAntivirus:
http://www.reedcorner.net/news.php/?p=341
* The expression ‘malware’ is a general term used by computer professionals to mean a variety of forms of hostile, intrusive, or annoying software.
You may find this User Tip on Viruses, Trojan Detection and Removal, as well as general Internet Security and Privacy, useful:
https://discussions.apple.com/docs/DOC-2435
The User Tip (which you are welcome to print out and retain for future reference) seeks to offer some guidance on the main security threats and how to avoid them.
Bear in mind that from April to December 2011 there were only 58 attempted security threats to the Mac - a mere fraction compared to Windows malware:
http://www.f-secure.com/weblog/archives/00002300.html
(I have ClamXav set to scan incoming emails, but nothing else.)