Yesterday my friend CALLED me over to fix her computer, an HP.

Windows XP
2.6 ghz P4
512mb DDR
geforce4 MX 440 64mb (now geforce fx5500 256mb)

When turned on, the hard drive spun and the motherboard made the regular POST beep, but there was nothing on the monitor. I switched out the video card for one of my old ones, and it worked PERFECTLY. The next day, she tells me the same problem came up so I TOOK home the video card and her monitor and tested them on my machine, and they worked fine.

I'm truly stumped on this one, anyone have any ideas?

Thanks for any help.Another slot for video card available?try re-seating the card again.Found this while looking for documentation to troubleshoot "Monitor is Blank" on my own HP Pavilion 6736 (yeah... I know it's outdated).  I haven't resolved my issue yet.  You MIGHT want to check HP site and verify it also applies to your friend's HP.

Documentation for HP and Compaq Desktop PCs:

Monitor is Blank after Starting the Computer:
    http://h10025.www1.hp.com/ewfrf/wc/document?lc=en&cc=us&dlc=en&product=61605&lang=en&docname=bph04760


Resolving Startup Problems in Windows 98, ME, and XP:
    http://h10025.www1.hp.com/ewfrf/wc/document?docname=bph07173&lc=en&cc=us&dlc=en&product=61605&lang=en


BIOS Beep Codes and Error Message Descriptions:
    http://h10025.www1.hp.com/ewfrf/wc/document?lc=en&cc=us&dlc=en&product=61605&lang=en&docname=bph07107