So I'll give the the generic answers to start till the details show up. (modem model?, how is it connected?)

1. Our preferred settings at http://forums.cnet.com/5208-7589_102-0.html?threadID=264135&tag=forums06;forum-threads

Use posts 2 and 3. Many of today's broadband modems include a router so the missing detail in your post has me note our sticky post.

2. Since it's Vista, consider disabling the VISTA BROADCAST FLAG. I find this fixes a lot of issue.

3. Again, the simple omission of the modem model number has me share nothing but guesses. Try this:
a. power off the modem.
b. move the ethernet cable to the next PC.
c. power up the modem then the PC.

Good luck,
Bob