Vista and Linksys/Vonage router

We have a linksys router that is vonage compatible (WRTP45G) and a laptop
with XP. We had the router phone jack linked to a phone wall jack so vonage
would be available in multiple rooms. We recently purchased a desktop with
Vista. Internet speed on the desktop is sometimes really low and sometimes
doesnt work at all but laptop does not experience these issues. Also the
phones no longer work through the house network but a phone plugged directly
into the router works fine. ISP gave us a new modem just in case that was the
problem.

Not sure how to fix the situation - according to Linksys the router is
compatible with Vista, Vonage tech support wanted us to use a fixed IP
address but then we had no internet connection. Not sure what to do to ensure
internet works consistantly.