IRQ's, interrupt requests, are how the CPU talks to every other component on the computer and how it avoids confusing what it is talking to or vice versa. Windows 9x and beyond mostly auto-allocate these. Sometimes it puts two or more devices onto the same IRQ which *can* work with some hardware.
As I read it, the thing that changed was Dx8.1 -> Dx9.
DirectX is how windows manages Video/Sound.
As a pure guess, could it be that maybe your system had a conflict that was manageable under Dx8.1...but Dx9.0 is less forgiving and now causes it to crash completely? There are some things you could try to troubleshoot this but I hesitate to describe IRQ's or PCI Steering if you've never worked with them before.
If you'd like to peek, try Start Menu - Settings - Control Panel. Open System Icon, Select Hardware tab and click on Devices button. Any red X's or Yellow Exclamation points indicates a problem that requires manual setting of device interrupts or can sometimes be solved by moving them around...aka, physically removing a card from a given PCI slot to a different slot. Yes, windows does do things differently based on which PCI slot a card is installed into. Obviously this wouldn't help with an AGP Video card, but if your modem/NIC is conflicting with your Video card, changing one of them might fix the problem.
Usually when you have video problems you'll get a blue screen saying vmm32.vxd (virtual device driveers) had an error.
I'd recommend my own primary learning method. Search the internet on topics and start reading.
www.tomshardware.com might give you some good background information.
Nax