Windows XP slowing down

[HTRednek]

Does anyone have a problem with Windows XP slowing down over time. I was used to this on Win 9X machines. I have XP pro installed of 6 of the 9 machines I own. Win2k on 2 of them. The W2k machines have run for well over a year with no problems but 2 of the XP machines that have the oldest installs are getting really slow. The worst one is a intel 815 mobo with 256 mb ram and a celeron 1 gz. I have defragged. scaned for virues and even turned off services not needed and turned off the indexing on the drive. The only guess I have is that the ram is unmatched. Some is PC 133 and some is PC 100. But the system ran fine on first install. Now it takes forever (compared to other systems) to start IE or outlook express. There are no programs running in the background or the tray other than NAV. Anyone with ideas would be greatly appreciated. I may just go back to win 2k as these are office machines that really don't need XP.