My phone has 560mhz procesor, hadly processing power of your pc. If you are having issues of this kind, maybe thermal sensor on your cpu is broken and cpu is slowing down because it things it has temperature problems, or bad drivers? That 1.8ghz celeron should be fine for majority of tasks. Maybe you could try downloading ubuntu live image and test there if the problem persists(its free)? Then you would at least know if it's not a problem with your operating system.

Funny you should mention thermal. Had to replace heatsink retention module recently. But that didn't help anything, really. It's just XP being all clogged up from years of use. Can't do clean re-install. Point of no return.