You need the correct driver for your graphics card. Do you know what card you have (even if it is an 'on-board' card built into the mainboard)?

Either use the driver disc that came with the card/mainboard, or find the correct driver on the web.

Usually the name of the graphics card pops up briefly when you switch the machine on - but it may flash by too fast to read, in which case you need to look at the chips themselves.

Email me if you need more specific help.