Don't take any of this as condesending...
It can vary from PC, but this is what you might see when you power on (all white text on black background apart from logos):
1) Graphics card info - chipset/bios etc. You might see something like NVxx (where NV is NVidia).
2) Next screen, maybe a Mainboard/Manufacturer logo (Asus etc) or the POST being performed - memory check, lists HDD/CD etc. You should see HDD LED activity and possible the floppy light on.
3) Next screen is like a "table" showing the components again, total memory and possibly IRQs etc.
4) Then the PC attempts to boot an operating system from floppy/CD/HDD etc depending upon the order set in the mainboard BIOS. In your case, I guess XP from the HDD.
Can you tell us what if any of these you see ?
PS Did you connect the PCI-e power cable to the GFX card ?
I don't think you need the connection to the ez-plug unless you are running 2 GFX cards (says Asus A8N-SLI Premium manual).
PPS Have you done anything like try to upgrade the GFX/Mainboard BIOS or change the Asus boot logo?