1. Did this PC previously startup normally untill you installed WinXP?
2. Do you hear the single short beep indicating successful completion of the POST, and then instead of the 1st display of characters you get a black screen?
3. Does all activity halt at that point?
Here's the appropriate point in the "BIOS Boot Sequence":
3. The BIOS performs the power-on self test (POST). If there are any fatal errors, the boot process stops.
4. The BIOS looks for the video card. In particular, it looks for the video card's built in BIOS program and runs it. This BIOS is normally found at location C000h in memory. The system BIOS executes the video card BIOS, which initializes the video card. Most modern cards will display information on the screen about the video card. (This is why on a modern PC you usually see something on the screen about the video card before you see the messages from the system BIOS itself).
This is rather mystifying but how about this for speculation:
a. If you had [in the default configuration settings] "Video BIOS Shadow = Enabled" in the "BIOS Setup->BIOS Features Setup" so that the video BIOS was being copied to a location in RAM and operated from there. [This is done to speed up the operations of the video BIOS. RAM works faster than the BIOS ROM.] Then if some other device was trying to use that memory location and had locked it, then your video card BIOS would be unable to function. Needless to say, you'd get no video display under these circumstances.
I wonder if the operating system is detecting the conflict and disabling the conflicting device, so the video works next time around.
When the PC normal startup succeeds the 2nd time around, go into the BIOS Setup and disable video BIOS shadowing and see if that fixes the problem. If it does, this fix will slow the operation of the video BIOS, so you need to search for the root cause [the conflict for the use of that space in RAM].