Performance Problems

I have a 2-computer network environment. One computer is a 2Ghz machine, the other a 266Mhz system. They are networked through a hub and the networking is done through Netbios with Microsoft.
The system also has an ADSL hi-speed modem connected to the hub, and shared by both computers. the BlackICE Defender program serves for firewalling.

After the computer has been running for about 4 hours, it slows to a crawl on both Internet and other applications. When shutdown and restarted, the speed is satisfactory. The system alos runs Norton System Works 2002, recently updated. BlackIce logging has been disabled.

Any idea or suggestion?

thanks

gilles mailhiot