change internet card?

There is no such thing as an "internet card".

There are ETHERnet cards, and as the name sounds similar and as they are often used to connect to high speed modems, peoples often confuse the terms.
A new ethernet card can be bought for under $20 in almost any computer shop.

If you have an integrated ethernet interface, you should disable it in the BIOS before proceding.
Reboot and enter the BIOS settings.
Locate the entry for the integrated ethernet and set it to "Disabled".
Save and exit the BIOS. Turn the computer OFF on the POST screen. You don't need to wait for Windows to load.

Anyway, you can replace the ethernet card as follow:

Turn OFF the computer and unplug it.
Open the left pannel of the tower.
Locate the ethernet card: there is an CAT-5 or CAT-7 ethernet cable connected to it.
Unplug that cable.
Remove the retaining screw and gently pull the card out.
Plug the new card in place and secure it in place with the retaining screw.
Reconnect the ethernet cable.
Close the case.
Plug the computer and power it up.
Windows will automaticaly detect and configure it, you only need to wait. It will probably take under a minute on the first boot after you install it.

Done.

Electro
December 2011