Question : can't open a web site using domain name on some pc's

Hi All
         a strange one, one of the sites i work on has there web site hosted off site. The site is up and running and i can connect to it from home and from some pc's on site. However there are some pc's that cannot find the web site when you type in the address but if you type in the IP address of the web site no problems.
If i do a ping to the site using, from these pc's using the www address i get a host not found error but if i ping the ip address it finds it no problems.
Now as i said this only happens on some PC's on site not all of them, they are all on the same domain and it does not matter whether i use IE or firefox same result. These pc's can get to other sites ok just not their internet site.
Any ideas where to look?

Answer : can't open a web site using domain name on some pc's

check to see if they are using the same DNS servers as the machines it is working fine on.

ipconfig /all

Make note of the DNS servers being used. While you are out there in command prompt land, flush the DNS cache for fun with:  ipconfig /flushdns

I would also check to see if they are resolving names properly with:

nslookup www.somedomainname.com