can you provide these infos:
Operating System and service Pack of your PC
Version of McAfee programs installed on your PC
RAM of your PC
If you installed a McAfee suite on your PC it should include virus scan already so no need for you to install virus scan plus that's why i asked if what's the name or version of the McAfee Security Center you installed. Is it Total Protection of Mcafee Security Suite?
If your are receiving an error that virus scan is not installed, there is a possibility that you have a conflicting progam/anti virus on your PC, kindly check the list of incompatible programs on this link:http://service.mcafee.com/FAQDocument.aspx?id=307274&lc=1033&partner=10005&type=TS
If you don't have any conflicting program, you just need to uninstall/reinstall the McAfee program on your PC by following these steps:http://service.mcafee.com/FAQDocument.aspx?id=107083&lc=1033&partner=10005&type=TS
After doing the said steps and tried reinstalling McAfee on your PC but your still receiving the error, do let us know.