Trouble Connecting to internet

My father recently got an old Dell Optiplex 740 desktop and has it connected with a wireless network adapter. His connection has been fine for the last few days, but today it has been giving us a lot of trouble. His web browsers won't connect to the internet despite his network reading "very good connection." I have tried using both Firefox and Internet Explorer and get the same results. Both browsers say they can't connect to the web server, despite what website I try to connect with.

The network itself is fine, I can connect just fine with my laptop (wireless) and another desktop (wired).

Trying to troubleshoot the problem myself, I found an extremely temporary fix. If I right click on the network icon and select "repair", I can connect to the internet, but his homepage (xfinity.comcast.net) will not load, but he can access his favorites just fine, and after that he can click the home button and comcast.net will load. If he closes the browser and reopens it, however, the problem persists and he can no longer load any websites.

His computer runs Windows XP (I am not sure which SP he has, but I imagine it would be SP3). His anti-virus is Norton (provided through Comcast). I already tried checking his firewall permissions and Firefox is permitted. I can't remember the exact wireless network adapter off hand, but I know it's a Netgear.