I format my pc and install win xp but i can't change the resolution fro 800*600 to 1024*768

Hello
Your going to need to install your chipset and video device drivers.Should have a cd or dvd from when you bought the pc...Applications and drivers?

Possibly running windows update will find them and install them for you.This method may take a while depending on your internet speed.

You dont have the drivers that is the problem that you cant change the resulotion from 800*600 to 1024*768
if you dont have the cd or dvd drivers when you bought the pc find the model of you motherboard if you have onboard video card if not see the model of the deticated video card, to take the model information you have to open the case of the pc, after that in google.com for <<"the model" video card drivers>> or just put the "model number" and i will post the link of the drivers.