ther is a smart freeware program
SparkleXP
SparkleXP is an application designed to tweak Windows XP and improve its performance,
it helps freeing up computer resources resulting in a faster and responsive computer.
This tool creates a second Hardware Profile and a correlative new User to get a lightweight desktop.
You can choose to boot into your regular or SparkleXP configuration at startup.
just sparklexp > sparklexp - or normalxp > normalxp.
its wery easy to use.
i used i when only got 1gb mem (2x512) in pair
after i bourght 2 gb (2x1204 in pair. and changed the mem from one to two gb i dont need to use i . ok it will still do it better but im lazy and do like to have my mail an other stuf on at all time,
and dont want to have mails recived some as one user and some as sparkle user.
another tip is if you want to play oblivion go get oldblivion and install it . after instaling oblivion . then just start the oldoblivion instad and like magic i can play it on my old nvidia fx5200 card ok sometimes i goes slow and i have to re install oldblivion , but it takes 1 minute and im up running again
hope this is usefull and sorry i im putting link here if its against the rules. its freeware and im too lazy to read rules.
http://www.sparklexp.com/