Make sure both Halos are the same version (1.0.5 I believe is the latest). Make sure you can get to the machine (i.e. make sure the PC can see the Mac and vice versa). The fastest way, I've found, is to turn on Windows File Sharing and on the PC in Internet Explorer type \\maccomputername\yourloginname if your PC prompts for a Password then all is well.
If not then I'd assume your PC needs TCP/IP enabled or it doesn't have the drivers installed for the LAN card. The Mac is pretty much foolproof on connecting to a network, though I suggest turning off AppleTalk unless you feel the need to connect to really old Macs. /Applications/Utilities/Directory Access. Any help with the PC networking can be found via google.
Oh and if your share name has spaces, they should be seen as hyphens on the windows machine.
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