If you've purchased your system within the past 2 years, it probably either came with the
Network Adapter integrated into the motherboard with a jack in the back of the tower, or
has a separate Network Adapter card already installed with a jack in the back of the
tower. Go to Device Manager, and see if there's anything in the Network Adapter category.
It doesn't have to say Ethernet it could list as a brand name Network Adapter and then
10/100 (same as Ethernet). If your system doesn't already have the above, then you can
purchase a decent 10/100 Network Adapter/Ethernet PCI card for about $15.