Right-click on the Taskbar and select "Task Manager". Open the "Processes" tab. See which process is eating up all those cpu cycles.
Are you running XP, Vista, or 7? Going off that model number it's likely Vista or 7. Under 7 I've had an issue with the "wmpntwk" process where it just eats your memory- thus causing your system to crawl. It's a process related to sharing media over the homegroup via WMP or WMC. Disabling it in MSCONFIG solved the problem. Wonder if yours is related.