2 years??? Hmmmmm . . . .
Suggestion #1 would be to turn simple file sharing off - this will enable permissions for the folders that are shared out on the MCE box to be better managed
Suggestion #1.1 - check that all of the computers on your network share the same workgroup name, DNS, gateway settings etc - makes NETBIOS stuff a lot more reliable when browsing the network
Suggestion #2 would be that, once simple file sharing is turned off, right-click on each of the MCE folders, select "sharing and security" and then click on the permissions button and add "EVERYONE" with full permissions (assuming that other people on the network won't go deleting files on you

). The other thing I find useful here is to also click on the "security" tab and make sure the same permission setup (EVERYONE / ALL or whatever you decided) is applied.
Suggestion #2.1 - consider creating a specific username and password combo on the MCE machine so that when the other machines connect to the shared folder, you can type in this username and password and so restrict the username that can access the shares. This also means that #2 becomes slightly different - instead of adding EVERYONE, you would add <USER> with "full control".
Reboot the MCE machine - then, from one of the other pc's go to the "run" command, type in \\<MCE computer name> and then double-click on any of the folders that should appear in an Explorer window. If it asks for a logon, try the username etc created at #2.1, and see how you go!!
Using this sort of setup, I have 1 x Vista box, 1 x MCE2005 box, 2 x XP boxes and 1 x XP laptop all happily talking to each other with no sharing issues (although the kid's computers are limited in what they can do TO a file as opposed to WITH a file!)
Any probs, let us know.
Cheers,
B.