You should also check the task Manager to see if any services are grabbing CPU cycles. Some Services and programs can severly slow down desktop performance without showing 100% CPU usage. You need to do somwe snooping to see what is happening. If you stop all work does the response get better or not? Doing the temperature measurement is critical.
XPS M1530, 2.0 Ghz Core 2 Duo T7250, 4 GB 667 Mhz RAM, 200 GB 7200 RPM Hard drive, Nvidia M8600GT Video - 186.81 Driver, Hi Def LCD - 1680x1050, Vista SP2
Inspiron E1705, 2.0 Ghz Core 2 Duo T7200, 2 GB 667 Mhz RAM, 160 GB 5400 RPM Hard Drive, Nvidia 7900 GS Video - 1920x1200 - WUXGA, Vista SP1