First: are you certain that your date and time settings are correct ?
If I ASSUME that you're not infected:
Usually McAfee updates automatically.
In the security center, there is an option to update or whatever it's called. Have you tried that ? (Maybe reboot, then when you get the message about the signature file, go to the security center, click on configure, select warnings and tick the appropriate box to ignore this 'problem', and then try the manual update ?)
Alternative, at your own risk: completely uninstall McAfee, using these instructions (from Ex Brit ?):
1. Uninstall all McAfee programs through "Add or Remove Programs" in Windows "Control Panel".
2. Use the MCPR tool, see this article: How to remove supported McAfee consumer products using the McAfee Consumer Products Removal tool (MCPR.exe). This will remove all McAfee remnants from your computer. (you must have Windows Vista, XP or 2000 SP4 to use this tool)
3. Launch Windows Explorer and delete all McAfee files in "Program Files" but especially in "C:\Document and Settings\<user>\Application Data" and "C:\Document and Settings\All Users\Application Data". You have to set Windows Explorer to show all hidden files and folders (Tools > Folder Options > View tab). Just delete the McAfee folders even if they are empty in all the "Application Data" folders. Can be more than one!
In Vista: C:\Program Files, C:\ProgramData and C:\Users\yourname\AppData\Roaming
4. Reboot and redownload/reinstall your McAfee products directly from your account at the McAfee web site, not from any CD that you may have.
You could also try removing the browser history, keep in mind that it erases form data, passwords etc.
Of course, it's possible that your system has been corrupted, and your best option may be a complete reformat of the harddrive and reinstallation of the operating system, drivers, programs, data etc. That should also work if you are infected with malware.
All suggestions at your own risk of course.