You might look to make sure you have a PCI slot available. Unless you have added stuff to the computer it is unlikely you don't have a slot. I think all Pentium 3 computers came with a PCI bus and slots.
It isn't hard to install. Take out the screw holding the blank plate and remove the plate. Snap the card in. Use the screw you took out to secure the card. It isn't rocket science. The hardest part is probably getting the side cover off the computer.
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