Im out of ideas

So I bought a new computer recently and decided to give my old computer to my youngest brother for his birthday, as it was-is still a pretty decent comp.
After getting all my old files off. and uninstalling some programs I didnt think My lil brother needed to be playing with (Dragon Age, ect), and checking that everything still checks out ok I send my comp to my parents and get it set up.

Tried hooking the computer up wirelessly, no surprise it didnt work. I had issues with wireless when I lived there 2 years ago. That house just eats up wireless signals :|

So dad finally gets around to running a wire from the modem all the way across the house to the computer... and it wont connect. They call me over, and I fuss and fiddle with it... and I get it to recognize that it's connected to a modem, but not the internet.

Frustrated I take it home with me and connect it to my internet where it does the same thing. A week ago it worked just fine. I've tried everything I can think of. All things seem fine, drivers appear to be working, the wire is new and works on my bro's old comp. Tried diabling and reenabling the drivers. Teh network devieses. Rebooting the comp (Stupid but eh..). Made sure the network card hadnt somehow gotten disconeected from the motherboard. Disabled firewalls, virus protection, adaware, ect.

I'm completely stumped. The only other option I can think of is that the network card somehow got fried between the last time I connected on it and the first time I tried to connect at my parents.

Anyone have any suggestions?
The comp is about 4 years old.
Is win XP

Edit:
On a slightly unrelated note. Does anyone happen to know for a fact if BestBuy thinks Firefox is bad?
My dad and I got into a fight over whether it was the cause of it all, because he thinks Firefox is the devils work, and has been told so by the folks at Best Buy. I personally think he's lying.