Trying to install a network card

I'm still on dialup and I want to get Roadrunner because that's the only high speed service available here. They came once to install but started griping about the fact I had Windows 98.
So I wiped my hard drive and installed Windows XP Professional. It works great, pretty smooth, easy install.
Now I'm trying to put in a network card to connect to Roadrunners cable modem. I was told that the USB connection is slower.
Every time I install a card and then turn on the computer the monitor has no signal. One time it even reset my bios and made WinXP not see my modem any more.
Is it wrong to put a network card in a computer that has a modem?
I build computers and upgrade them regularly, but I have no experience with network cards.
Here are the specifics on my computer:
Compaq Presario
AMD-K6 3D Processor
533 MHZ
184 MB RAM

Anyone have any ideas?

Maybe the network card is using the same IRQ than the video card.

Older computers were not plug and play, we have to tell them what they must use.

Try to find in your motherboard manual something about IRQs and reserving them to specific things.

Ar is right.
You should also try removing your modem-dialup.
It is not uncommon to have issues if you have both.

hope that helps.

Both comments were very helpful, thanks!
I just never had a computer act that way after I put in a new card.
I wonder if the installer guy will be able to do it for me?
My computer is sitting here without it's case so it shouldn't be a problem for someone who knows what they're doing, should it?