|OS: Windows XP|
CPU/Ram: Celeron D/256mb
That 256mb RAM might have a lot to do with it as WinXP uses about 240mb RAM to start up leaving you with 10mb for internet browsing before having to use the page file that will slow you way down.
I would recommend adding as much RAM as your machine will allow.
www.crucial.com has a Wizard to help you.
Otherwise, I would set Internet Explorer's Tools, Internet Options, Advanced, Security to "Empty temporary internet files folder when browser is closed" and close it every so often when things start slowing down.
Additionally, others may offer more.