To add to what mende1 said, make sure you use the uninstaller to remove the anti-virus program. Sometimes just trashing the application leaves other parts of the program in place where they have the potential to cause problems.
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What's the message you see when you open the app? Have a look at this website > http://support.apple.com/kb/HT5290?viewlocale=en_US&locale=en_US
OS X doesn't need any antivirus, and if you want one, you should get ClamXav. Other ones only make the computer slower and may damage Mac OS X. See > http://www.thesafemac.com/mmg