Playing back MPEG would be fine if the video is low resolution, such as VCD quality MPEG. However it also depend on your OS, Windows XP probably will use up all the computing power already.
Avi and wmv are more advanced compression codecs. I don't think your computer would handle it.
Popular codecs include DivX, Xvid as avi files require huge amount of computing power. It nearly lags on my Athlon 1.2Ghz CPU
rm requires a bit less processing power because realplayer has an option to decrease rendering quality; however don't put much hope that you can play it.
Same goes with mov files.