OK, Gaz, couple of things. The fan you got with your CPU was presumably 'bundled', which means it was any old fan that would fit a Socket A mobo and on paper ought to cope with chips up to the nominal speed.... Ha. It's all about power, heat and airflow. You may need to change the CPU fan, but not yet.... Since you are OK (if rather warm) with the side panel off, and overheating with it on, then something is not right with the airflow inside the case. There is at least one socket and often two for case fans on your motherboard (or some fans power off PSU power leads); think about installing one at the front (often mounted under the floppy) blowing in, and one at the back, blowing out (Yes, we've all put them the wrong way round before, so if you can't tell from looking at it, see which way the air goes). This should help airflow and internal cooling. Also check that any vacant PCI slots have closers in (don't leave holes). You should also check the power supply - if this is running hot, it will make everything else run hot, and hot is a sign of approaching failure with PSUs. If you still have a problem, then upgrade the CPU fan.