|I can't image that your laptop was "fast" with just 512MB RAM. I'm sure it ran OK, but fast?|
If your CPU is running hot, it will automatically lower the clock speed as a way of lowering the temp. When's the last time you blew out the cooling vents with compressed air?
As for the reinstall of XP, did install the latest drivers for the chipset, video, audio, LAN, etc? Did you install SP3, IE8 & all the current Windows Updates? Did you install a lightweight anti-virus program such as Microsoft Security Essentials?