You cannot install Windows Live OneCare and Forefront Client Security on the same computer

CAUSE
Windows Live OneCare and Forefront Client Security are anti-malware products. Therefore, you cannot install both products on the same computer.
RESOLUTION
If you want to change the anti-malware program that you use, first uninstall the anti-malware program that is already installed. Then, install the anti-malware program that you want to use.

For more information please visit the KB artilce: 946265