Computer help: Internet Connection Stops Approximately After 10 mins

Internet Connection Stops Approximately After 10 mins

Hi.
I'm really annoyed with this problem
I run Windows XP Service Pack 2
My startup programs :
ZoneAlarm Firewall
AVG Anti-Virus 8 Free Edition
Free Download Manager
avast! Antivirus (I've just installed it, planning to uninstall it later)

The problem is well... my internet connection stops every ±10 mins
I can't surf the internet, I can't play online games, except the one with local private server (Counter Strike 1.6).

I use 2 computers and share internet connection by multi-port modem.
The other computer is okay. Not a single problem about the internet connection.

I use the same browsers, Mozilla Firefox 3 and Opera 9.6

I have scanned my computer using 2 antivirus (As mentioned above, AVG and avast), but no virus found (Only Tracking Cookies found and I removed them and It seems tracking cookies don't have anything to do with it, I guess)
I have checked msconfig for malicious programs, found 1 program suspicious, removed it from the Service List, no effect.

Does anyone know what cause such problem?
Thank you so much for replying