Network connectivity problems, but *only* on my laptop

I’ve tried everything I can think of, and still nothing. All of a sudden my laptop cannot connect to the wireless network; it hits the router and then says that it cannot connect to the internet. The router is known to be good. I’ve tried physically plugging in a known good Cat5 cable, and the transfer is slow enough that it won’t connect to any sites despite saying it’s connected (e.g. sent 100, received 1 and barely goes up from that).

No other computer in our network is affected and is able to access the internet with no problems. It was working fine a few hours ago, but now nothing.

I’m out of ideas on how to fix this, and it’s been several years since I’ve done any real network administration. Any suggestions?