Problems...

[Genesis15]

My uncle wanted me to fix his HP Pavilion. And I guess when it starts up it goes through the HP screen then it turns black with a blinking _ one of those in the top left of the screen. Nothing happens, I popped in my XP CD and set it to boot off CD-Rom and nothing comes up saying Boot of CD. I tried putting the HDD in MY computer to reformatting and it wouldnt boot off my HDD it wouldn't recognize eigther of them. Then it messed mine up so I hate to reformatt my computer. Any ideas on how to fix his computeR?