Can't access local sites

OK, this has me stumped. Just got a new laptop with
Windows XP Pro, and here is the problem I have it and
another laptop running XP Home connected to a wireless
Linksys router accessing my local lan. I host web sites,
and my local lan has multiple servers, etc. From the XP
Pro laptop, I cannot access any sites hosted locally on
my lan, as in I cannot access http://www.domainname.com.
But if I enter http://domainname.com, then I access them
just fine. Or any other non-www host name. It's only
URL's that are local and begin with www that cannot be
accessed. The rest of the internet is fine.

Now it also had norton anti-virus and Norton Internet
Security installed. I completely un-installed Internet
Security, and that didn't help. Also, I know it's not
the router, as the other laptop attached to the router
does not have this problem.

Any ideas or suggestions?

Thanks in advance to anyone who can help!