Alan, I wouldn't immediately suspect your performance issues are being caused by overheating. The two biggest culprits are a lack of RAM or a rogue process that's constantly using 100% of the CPU.
As you've got 3GB of RAM installed then it's probably not a lack of memory that's causing the problem. To troubleshoot the second likely cause, open Task Manager and click the "Processes" tab. If available, tick the box to "Show processes from all users", then click the "CPU" column heading so that the most active processes are at the top. Do you see a rogue process that's constantly hammering the CPU?If you do then post its name. Ignore the "System Idle Process" as that's the opposite of busy.
It would be a great help if you let us know the version of Windows you're running.