Ahh the Dell D820 - I know it very well, we use Dells at my workplace. The honest answer is: I'm not sure. My guess would be it's overheating. That laptop is a few years old now and I'm wondering if either the thermal compound connecting the processor to the heatsink is starting to fail, or it's simply starting to get full of dirt, hair and general crud that laptops and PCs hoover up over time. This is normal, all laptops will do this after a few years. If the laptop starts to overheat then the laptop will slow itself down to protect itself.
This is conjecture by me though. We have a support contract with Dell - if one goes wrong we simply send it back to a Dell engineer to fix, we rarely open them up to fix them ourselves.
Anyway, I'm waffling. First step: to rule it out being a softare issue, first thing I would do is back up all of your data, wipe the harddrive and reinstall Windows XP from scratch.