Sometime network doesn't work

I have two machines on my home network, one running Vista (x32) Ultimate and
one running XP SP2. Every once in a while when the Vista machine boots, I
can't access the Internet. It's strange...I get an IP address from my
router (it's static since I use the machine as a quasi-server) so it's
making it that far...however I can't get anywhere outside...I can't ping
beyond the gateway...ifconfig /release / renew doesn't make a difference...I
basically have to reboot the machine again...then everything is fine.

Any ideas what I should be looking for? I do have a firewall running, but I
don't think that's the problem as a simple reboot usually clears things up
(i.e. I'm not changing anything to get the networking functional).

I'm really at a loss. Is there a way to "restart" the networking piece of
Vista once it's booted? There doesn't seem to be a catch-all from the list
of Services and I don't know if the "net" command will do it (or maybe I
should say I'm not aware of the syntax that will completely stop and
restart).

Thanks for the help!

-Gary