If you have Macafee virus scan, there is a bug with certain versions that hose up your PC while Macafee is downloading updates from the internet. It causes your CPU to spike. When it's running slow, do Ctrl + Alt + Delete, then choose Task manager. On the Processes Tab, look for Vshield.exe or similar process that is taking up lots of CPU. If you see that, you'll know you have this bug and need a newer version of Macafee.

Mine runs this update at about 10:30 PM each night, which is prime time for trying to surf the net. Is there a way to change the time of the updates with McAfee??

McAfee VirusScan 4.5.1

1. Double-click on the VirusScan Console icon in your system tray at the lower right corner of your screen (the icon looks like a magnifying glass with a small clock on top of it).
2. Double-click on the 'AutoUpgrade' item in the list.
3. Click on the 'Schedule' tab in the window that opens and check the 'Enable' box.
4. Choose how often you want to update your program (generally it is a good idea to update at least once a week).
5. Check the 'Enable randomization' box to make the update run at a random point within the specified time window after the designated update time (this step is optional, though it reduces the load on the update servers by not connecting at the same time every update).
6. Click the "OK" button at the bottom of the window.

McAfee VirusScan 7.0

1. Right-click on the VirusScan icon in your system tray at the lower right corner of your screen (the icon looks like a blue shield with the letter 'V' emblazoned on it in red).
2. Click the 'VirusScan Console...' option from the menu.
3. Double-click on the 'AutoUpgrade' item in the list.
4. Click on the 'Schedule' button from the window that opens.
5. Check the 'Enable (scheduled task runs at specific time)' box.
6. Click the 'Schedule' tab.
7. Set the time that you want to schedule the task, as well as the frequency (generally it is a good idea to update at least once a week).
8. Check the 'Enable randomization' box to make the update run at a random point within the specified time window after the designated update time (this step is optional, though it reduces the load on the update servers by not connecting at the same time every update).
9. Click the "OK" button at the bottom of the window.
10. Click the "OK" button at the bottom of the window to exit out of the AutoUpdate program.

Also, like others have said, open Task Manager with ctl alt del and look at the Performance tab to see overall CPU usage and then the Processes tab to see what program is running high CPU. By clicking on the CPU header, you can sort them in ascending/descending order.