When connecting via router, some sites won'tt open

Hi,

I've been slaving away at my parenys' house, trying to set up a network
for them. I bought a DLink 604, and hooked them up. At first glance,
all was well, but then I noticed that IE will not open some sites
(microsoft updates, download.com, occasionally google, and so on),
while other sites, like Yahoo!, open well. At first I thought the
problem was on one machine, then I discovered it on both.

I reinstalled the PPPoE software on one machine, and reconnected it to
the net (I uninstalled the protocol as per the instructionsfor the
router, but did not remove the dial-up connections so it'll be easier
to revert back. I wonder...). All the sites work fine. So I'm
thinking this has something to do with the router.

Setup summary - DSL modem, DLink 604 router, two machines running XP,
pro SP1 and home SP2.

I read some previous threads, and saw a similar problem to mine, but
not similar enough...

Any ideas?

Thanks!