Floater's Guide on how to install a new video card.
Step 1: First unplug your tower and remove any cords that are plugged into it.
Step 2: Remove the case from the computer, it should be screwed in at the back.
Step 3: Touch the metal frame in order to get rid of any static electricity.
Step 4: Find the proper slot for your video card. Most likely it will be PCI-Express of AGP, sometimes it will be PCI.
WARNING:Do not, I repeat do not touch the gold contacts on your video card, this could damage it severely.
Step 5: Insert the video card into the slot, be careful not to scratch the gold contacts, even the tiniest scratch could ruin your video card.
Step 6: Close your case, reinsert your wires, boot up your computer.
Step 7: Install any needed drivers.
I'm pretty sure I covered it all, if you need help with anything feel free to ask.