Internet connection working, but applications can't access the internet?

I came home from a stay in the hospital to find my computer can't access the internet anymore from my applications (like Firefox, Trillian, etc, and my Firewall and Virus Scanner both want to update but can't...)

I checked to make sure everything was correctly plugged in.

My sister's computer is connected to the same modem, and her internet works fine.

I went to Network Connections and it says that my computer IS still connected to the internet (able to send and receive packets). I tried to "Repair" the connection, but nothing changed.

I even tried to do a system restore to a point where it was working, but that didn't change anything either.

I have Windows XP, with AVG Virus Protection and Sygate Firewall. The last program installed was iTunes.

Internet connection working, but applications can't access the internet?adware

woa ur moving to fast,, there are other steps to take before u do a system restore,, if ur connected to the same modem as ur sister can u ping her computer? is ur DNS client functioning? many applications will error out because they call for DNS resolution rather than IP addresses if ur DNS client is down which happends sometimes on windows boxes it may need to be refreshed,, in ur network connections, are u reveiving and sending packets? where does ur IP address come from? are u using internal DNS/DHCP on ur router? i dont think ur firewall did anything unless u told it to so i wouldn't start there,, it sounds like a general addressing issue as to where networking should be completely restarted.