Did you previously have any security software before you installed McAfee?
Did you previously have any signs of virus or malware?
What type of operating system are you using?
How do you connect to the internet?
You could try the McAfee Virtual Technician. MVT will scan your computer for any McAfee related issues and if it finds any it will attempt to fix them for you. MVT can be found at the top of this page under Useful Links in the Technical Support link.
You may also have some corruption in your installation. Try to uninstall via Add/Remove programs and reboot. Next step is to use the MCPR removal tool found at the top of this page under Useful Links. MCPR will remove any remaining traces of McAfee that fail to completely remove via Add/Remove and reboot again.
Follow the instructions in this link http://service.mcafee.com/faqdocument.aspx?id=TS100119 and be sure to use the McAfee preinstall tool found in the link before logging in to your online McAfee account and downloading your software.
Let us know if this works for you.