Make sure both computers are on the same workgroup. The Mac workgroup name is called workgroup by default. Sometimes the PC is on MSHOME by default.
Also a router can make it a little more difficult, but if both computers are on the internet using the same router, then both computer should be able to see each other.
If you want to be able to share your iTunes Music, on the Mac side you click on the iTunes Menu, then select Preferences, then click on the Sharing section. Then check the box that says share my music. Or, if the music on the PC, check the box that says look for shared music.
In iTunes for Windows, click on Edit, and then click on preferences. Then click on the sharing section, then click on either look for shared music if the music on the Mac, or check Share music if the music is on the PC.
If the router isn't blocking anything, the shared library should appear on the opposite computer.