Computers lags (Only in video-related tasks)

At least that's what i'm noticing.

Here's the issue, subject: Compaq Presario 5650LA
PIV:2Ghz.
1GB RAM
Videocard: Ati all in wonder radeon.
OS Windows XP: SP2.

Every time i start a task that needs some sort of "advanced" graphic commands (Read: Games, emulators, even running videos on media player classic) the video/programs starts to lag...to the point that the picture is frozen, the sound suffers the same problems...and it takes a while to close the aplication.

Additionally, the fan sounds more loud than usual, it's like....it's at max capacity 100% of the time (the sound starts after executing a task in windows, and stays like that)

I updated ATI drivers, no good, scanned for virus and spyware, same result,  i checked inside, to make sure everything was installed and connecter firmly (mainly the RAM), no good, i used scandisk/chkdsk and defragged teh volume and is still the same.

Note that i had this problem before, formatted, and it's still there (I don't recall how or why it started, i noticed the issue some months ago, i installed Age of Empires and to me, it seemed weird that such an old game lagged with my current computer specs)

Any suggestions? need more info?

I was just running in hardware acceleration disabled mode and everything was slow as shit, with videos freezing up everything and other stuff. Just a thought..