I have a problem of the same nature.
I recently purchased a LinkSys ethernet card for my Dell Dimension 8200. I READ the instructions and then installed the card into one of the availible PCI slots. I then tried turning my PC on and guess what? It would not do anything. However there was power going to the mobo cause the litle green light went on. Pressing the power button did nothing. I removed the card and the computer powered on with no proble. I tried it in the other three PCI slots and it did the same thing. I then got mad and pressed the power button and plugged the dam card in and it kept booting and windows detected the card and intalled it. However, when ever i turn the PC off, I cant power it back on unless iremove the card. In my BIOS IRQ assignments, it shows the card as "unknown manf ethernet card". I have updated the BIOS to the most recent version and the problem still exists.