I tried putting Linux (Ubuntu "E") onto a Toshiba 480CDT that was running Win2K reasonably well (233MHz PII, 172MB RAM).
It was painfully slow under Linux. Usable with Win2K, not usable with Linux. So go for the "lightest" Linux you can use. And good luck with finding wifi drivers!
You should be able to run Win2K nicely with the Toshiba Satellite 4600 - but only if you increase the memory (I'm assuming it's still 128MB).
I've run WinXP on desktop PCs with 600MHz PIII CPUs - but with at least 256MB RAM, and usually 512.
The other thing is - how recently has it had a re-install? A system that old is likely to have slowed down somewhat and would undoubtedly be quicker with a clean install.
SD