1. Disable 'Network Discovery' in the 'Network and Sharing Center'
** Control Panel\Network and Internet\Network and Sharing Center **
2. Disable AutoTuning:
Open up a command prompt with Administrator Priveledges (go to the start menu, Accessories, and right click on "Command Prompt" and select "Run as Administrator").
At the command prompt, type:
netsh interface tcp set global autotuning=disabled
(note: this will be reset upon reboot)
3. Disable Stateful Packet Inspection on your router.
If this doesn't work, consider purchasing a new router or extorting a Vista compatible revision (3CR858-95) from 3com.
Your router has been discontinued as of February 1, 2007, probably in light of compatibility problems.
3Com product End of Sale Statement