4 GB minus everything else like the video card, etc. Depending on your hardware, that leaves 2.5-3.2 GB available.
Stupid question here - I'm only showing 2.99 gb on My Computer Properties. So that's the amount after the machine takes away what it wants for video memory on a laptop, and any other things? I have a Toshiba Satellite A05-S4284 running XP Media Center edition.