Video runs slower or won't play

First, the basics - Dell2400 2.3Mz Celeron, XP,1GB ram, 40GB HD, on board video card.

Nothing fancy, but a nice little setup. For years I have had no problem playing any video files I had. Sometimes, one player would play one type better than another, but never had a problem, usually with Win Media, and running full screen. I try the files on several different players, and none of them work smoothly anymore.

Now, I can't get video files to play without stuttering or running real slow. Audio works fine, but video lags behind. Have tried different players, run updates, updated drivers, run virus and malware scans, everything I can think of. It is just as if the computer is suddenly to under-powered to run video files, even those it ran fine before. Sometimes I can get a file to run fairly smooth in Win Media at the default size, but as soon as I go full screen, it starts jerking and slowing down. Sometimes it is almost like a slideshow. There does not seem to be anything running in the background that would slow down performance, or that I haven't always run before.

I know my setup is capable of running these files withoit problem, but now they run so slow and with so much problem that I can't stand to watch anything.

Suggestion?