hi do you mean 800 x 600? and could you tell me why you think you need to use this particular resolution (if it exists). if you add a better graphics card you will be able to reach much higher resolutions. the instructions below are for xp you didn't give any details at all.
if you right click on the desktop and go to properties, then click on the settings tab. click advanced then go to the adapter tab. click list all modes and you will find the full list of settings your monitor can run with whatever your particular display adapter is.
you could try keying the make and model of your monitor into google, you should be able to get info on the range of resolutions you should expect to get with it. if you're unable to display any of the resolutions listed it's down to the graphics. if you have onboard graphics you could try upgrading to a pci or agp graphics card.
let me know how you get on