Display goes blank whenever booting into a GUI; infinite loops with some software

Here's the PC.

I posted something similar in Software about the issue, but as I'm working on this, I'm beginning to see it's certainly hardware related.

Summary:
The computer had a trojan (desote.exe), so after fruitless efforts to remove it, I erased the hard drive and reinstalled Windows (XP Home). When the install finished, I saw the record for the last installation of Windows didn't get cleared from the MBR because I used the format option on the CD. It didn't matter, because the screen would go blank every time I selected the "correct" installation. I ended up flattening it with DBAN.

Since then, anything and everything that has any sort of a graphic or graphically represented text will result in a blank monitor. Booting from a Seatools for DOS disc, for example, will display the process log all the way up to when it officially launches the "seatools.exe" program, and then the screen will go blank. Ubuntu does the same. I can never complete the Windows installation because the second part of the installation (after the first restart) is through a GUI, and the display won't show-- the first part, however, does show.

This is a bizarre issue, and I've done the following to try to resolve it: zeroed out the HDD; cleared CMOS by jumper/battery; removed all cards, including the video card, and used the onboard video; checked the POST status, which passes.

There is no Windows installation for me to work with, so no booting into Safe Mode is possible. No diagnostic program that uses a GUI will start up (simple linux- or DOS-based ones like DBAN will).

What causes a computer to selectively display only screens with text or BIOS info?

Some things are intermittent: the Dell logo and POST progress bar occasionally show, although I usually just have to guess when I have to press F2 to get to the BIOS menu, which is always available. I have inexplicably seen the Windows loading screen a couple times, whereafter it prompts that "setup is restarting..." and then the computer restarts, with or without the OS disc in the tray.

The only thing I can think of is a fried motherboard, but I really don't want to tell my client that I've had her computer for this long to tell her it's dead, because it was at least booting to the virus-ridden Windows before I took it with me.

Any ideas are appreciated! This has me stumped!