Lan card conflict

Hi,
I have 2 lan cards in my desktop (winxp pro sp2)Onboard & a secondary. The onboard one is not workin. The secondary one is Scillian product. The problem is with my internet. I use cable net with its own dialer. I get connected to the internet but only for a fw mins. After that I get PCBD for any site that I go to. I have ccecked for antivirus, spyware & adware. Pc is clean. I have formated my Hard disks 4 times & reinstalled OS. But no luck. The last time I formated my disk I have loaded only the OS & no luck also. I have also disabled the onboard lan card through BIOS also but no luck. I am able to connect to the internet on my laptop useing the same dialer & do not face any issues.
NEED HELP DESPRATELY…..(planing to through this pc & buy a new one if the issue is not resolved. 🙁 )