That only happens after a major update to the Security Center itself. It's wise to reboot immediately to avoid the pop-ups. It isn't uncommon for updates to just about anything to ask for a reboot from time to time.
You can control updates however. Double-click the taskbar icon to open SecurityCenter
Click Advanced Menu (bottom left)
Click Configure (left)
With SecurityCenter highlighted at top left
Click Advanced (right)
Click Update Options (left)
You can modify settings over at the right.
If it occurs repeatedly then something is wrong and you should try the following:
You don't state what products you have installed but I'm assuming that it is Home/Home office.
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 (MC... 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
If you can't find these files don't worry. It just means that MCPR has done its job.
4. Reboot and redownload/reinstall your McAfee products directly from your account at the McAfee web site.
If you want to get a refund, McAfee offers full refunds within 30 days of purchase. Either take the software back to where you bought it, if on CD, or contact Customer Service, link at the top of this page, second from the left. You can phone them or use online Chat.