If its a driver issue, he'd get video display at boot/post/pre-windows loading screen and he'd also be able to get into safemode by pressing f8 when the system is loading.
Vozzy, if you can't see anything displayed on the monitor when you boot the pc with the monitor connected to the new card, then this is not a driver issue. If the display is fine until the windows loading screen, then it goes black, then it is a driver issue.
If its a driver issue, you should first login via safe mode by pressing f8 during boot and selecting safe mode from the options listed. Uninstall any ati drivers you have loaded, reboot. The system should boot and work at reduced resolution with the default video drivers in windows xp, then you can try the driver recommended above if that works.
If you aren't getting any video output at all with the new card, there are only two possibilities I can think of. Misconfiguration in bios, or a defective video card. If you were using an external video card before or a different external card works fine, then you don't need to change bios setting and the new card should just work. Originally you sounded like you were only using the onboard motherboard graphics, but your last comment sounded like you've used another card without any problems.
Give me some more details and we can give better instructions.