Probably heat issues, trashed OS, or HDD failure.

Back it up like you plan. Then take an air compressor or compressed air cans and clean the living daylights out of it. If you're afraid to open it up yourself, make sure you direct a shit-ton of air through the intake to move as much dust as you can out the exhaust.

If that doesn't improve things, start up a command prompt as Administrator and run CHKDSK. Fix any errors reported. Reboot and see if things improve. If they don't, try your reload/factory restore. If the restore fails, or your symptoms occur during the restore process, you probably have a bad HDD.

If all else fails, follow the devil down to Georgia and PM Byte.