XP Pro, ICS, DNS Resolution Failing

Hi there,
One of my clients small networks consists of 4 PC's. One
PC running XP Pro has a dialup connection to the internet
and is sharing the connection to all of the other PC's,
its acting as the gateway / file and print server.

Two windows 98 client machines access most websites fine,
except certain ones like microsoft.com or xtra.co.nz and
none can sign in to the MSN messenger service. Cannot
ping any of these sites at all. But other sites like
google.com are fine??? The XP machine can with the
connection can access all sites ok.

Are there any knowen problems with DNS Resolution through
ICS on XP Pro??

Thanks! Need your help!