Heres a good one from a friend of mine
I recently gave him an old PC as a favour so his girlfriend could study while he worked on his own PC. I had always had trouble with the machine crashing and made this known to him when he took it. He installed a new HD and some new memory and it seemed to wrok great.
Trouble is now he is setting up her new machine to run on thehome wireless network by installing a card into the PC, but the PC does nothing but crash with the card in. Remove the card and all is fine again.
Can anyone suggest anything?

Usually (not always) wireless cards or any cards require you to install the drivers before inserting the card, so I'm assuming this has been done. You don't say at what point the crashing occurs: powers up with blank screen and stalls, will not boot, boots but crashes before desktop, crashes after desktop loads, crashes upon insertion of card, crashes when attempting connection to router.

Shooting in the dark, I would suspect a hardware conflict of some kind, possibly an IRQ conflict. Also, if it's an older machine, the hardware architecture may not be compatible with a newer wireless card. Checking the machine/motherboard manufacturer for an updated BIOS may produce a solution. Good luck; hardware problems can be the toughest boogers to track down.

I'm inclined to agree with Neophyte. It sounds like a hardware conflict.

Depending on the age of the machine, sometimes it helps to switch the card with another PCI card in the machine. Some motherboards are picky that way.

First install driver and then plugin the new hardware