^^^ yep.
While an adapter and a router can both advertise as 1Gbps, network cable distance and quality can drop that back to 100Mbps in reality. My home LAN is like that. I get gigabit on my desktop where our router sits on my desk. My wife's PC is connected with a 25' piece of cat 5 and it only gets 100Mbps even though both router ports and both Ethernet adapters support the higher rate.
Regular cat5 won't do gigabit, could also lose gigabit if it's not crimped properly, or there's a wire busted.
How expensive are ethernet cards? Could I perhaps just try swapping it out?
If swapping the cable didn't remedy it, I'd almost say that it's a NIC issue. Since it's your desktop with the issue, you could always grab a PCIE 1x nic and try it out.
I have one of these, driver is easy enough to grab since it's right from Intel (was on purpose).
http://www.amazon.com/gp/product/B001CY0P7G?psc=1&redirect=true&ref_=oh_aui_detailpage_o00_s01
Any cat5e/6 cable will do, just make sure it's copper and not copper plated aluminum. I wouldn't spend more than a few $ on one though. Probably going to be $10-15 if you're picking it up at a store though.
edit: originally read that as cords, my bad.