Well, there are a lot of possible reasons for that. Are both computers properly connected to the network? Can both ping others for example? Then, are you getting the IP right? And is there anything that makes that unpingable computer different to the others in any network related way? (e.g. different OS, different security software, different type of network card,...)
Oh, and please read the
Posting Guidelines, especially the section
Use a Good Thread Title. (Because "Hi!" isn't a good title for a windows networking question.) I'm renaming this thread for you, so that it can catch more attention.
Greetings,
Nepomuk
