no video signal

We have 2 computers running Windows xp pro that have developed the same problem about three weeks apart. The computers seem to boot up, getting one beep so I believe they're posting, but the monitor screens are blank and a no signal message shows momentarily on the screen. One of the computers has on-board video and an agp video card but neither output works. I know that the monitors are working because I connected them to my third computer running Windows 2k and they are fine. We've reseated the video cards and all the fans are running and the computer seems to be fully powered up but there's no video. Could this be a virus or some hardware problem? Both computers have msi k7n2 motherboards, one with a athlon xp 2500 cpu and one with a athlon xp 2600 cpu, both with 512 mg ram, both have a sapphire radeon 9800 pro video card and one also has onboard video.

 

Hehe, I just have to point out that I have almost the exact same computer.

Athlon XP 2500
MSI K7N2
512Mb of RAM
Sapphire Radeon 9800 Pro

 

9800 pro has 2 Vga out one is CRT and the other is DVI, so I think someone has been changing it to other one in Display Property