My bf's a computer tech. I read your post to him. Here's what he said:

You are having an IP address conflict between the two computers. Here's how to solve it:

1. Go to the XP machine and hit "start" then "run." Then type "cmd" in the window that pops up. A black screen will open

2. In the black screen type this exact phrase: ipconfig(space)/release

3. In the next line you type: ipconfig(space)/renew
This will give you a new IP address.

4. Now shut down both computers, shut down the router, and the DSL.

5. Restart the DSL first

6. Then the router

7. Then the XP machine

8. Then the vista machine

9. And the problem should be solved.

If that doesn't fix it, his guess is there's a problem with the router itself and he suggests you call tech support for the router.