It is a computer browser issue.and a possible IPv6 issue.  The Mac wants to
dominate the network and doesn't share the comuter names the same way that
windows does.  If IPv6 is active on the Mac I think it also distributes the
workgroup names on the IPv6 protocol which the windows computer, unless you
installed that protocol, simply does not "understand" what the mac is trying
to tell it.  I don't know why the browser functions hide the PC's sometimes.
Here are some sites for you to look to for teaking the settings on your mac
to make it play better with your PC.

http://www.chicagotech.net/browser.htm#Q:%20How%20can%20I%20check%20my%20Master%
20Browsers

http://joelshoemaker.com/computer/mac/pcfilesharing.html

http://homepage.mac.com/william_white/smbdoc.html#_3._Diagnostic_Tools

http://www.macosxhints.com/article.php?story=20031113060738627&query=file+sharing

Let us know if these don't help.  If one of these links dumps you at a forum
or site's main page and not a specific message make sure you search for
things under "computer nrowser" and not simply file sharing.