WindowsXP Pro&Windows 2000 Pro network

As you didnt submit the details but i wish i would clear all the respects related....

First About your Router :

The Linksys EtherFast? Cable/DSL Router with 4-Port Switch is the perfect option to connect multiple PCs to a high-speed Broadband Internet connection or to an Ethernet back-bone. Allowing up to 253 users, the built-in NAT technology acts as a firewall protecting your internal network.

Configurable as a DHCP server for your existing network, the EtherFast? Cable/DSL Router with 4-Port Switch acts as the only externally recognized Internet device on your local area network (LAN). The router can also be configured to block internal users' access to the Internet. A typical router may have to rely on an external hub or switch to share its Internet connection, but the Linksys EtherFast? Cable/DSL with 4-Port Switch Router channels this connection through the blazing, full duplex speed of its built-in EtherFast? 10/100 4-Port Switch.

This cutting-edge combination of router and switch technology eliminates the need to buy an additional hub or switch and serves your network as a completely dedicated, full duplex backbone. Now your entire network will enjoy blazing Broadband Internet connections supported by its robust switched backbone. With the dual-function speed and power of the EtherFast? Cable/DSL Router with 4-Port Switch, your network will take off at speeds faster than you ever imagined possible.

So The Possible aspects were:

The router can also be configured to block internal users, so check the configuration if you do such thing to block the communication with each other...

http://support.microsoft.com/default.aspx?scid=kb;en-us;306541

See the above article. You have a peer-to-peer network going... the LinkSys is DHCP, but you might want to set ip addresses statically (for consistency between the two computers).If both computers cn access the web, then LinkSys is fine and the computers are fine.

It's only a matter or configuring the user accounts properly and allowing access. The above article should help.

1. Ensure all machines on the network can ping each other by IP address.
2. If they can ping by IP, then you need to ensure that all of the machines are in the same workgroup.
3. If all are in the same workgroup, wait about 30 minutes and all machines will be in the 'My Network Places' using the Windows Explorer.
4. Also ensure that all of the machines are using the same DNS domain.