There are several methods to sharing your iTunes Libraries across computers in your local network. The easiest method of them by far is for connecting any Macs in your network, connected by Ethernet or WIFI connections.
Start iTunes on the computer with the Library you wish to share.
Go to Preferences. Select the Sharing pane, and check the "Share my library on my local network" box. Along with checking "Look for shared libraries" for good measure. You can then decide if you want to share your music, movies, etc. or all of your Library.
Now, on your other machine, start up iTunes. You should be able to see the username of the computer with the shared Library in the left side along with your Library and Playlists.
If you do not see the Shared Library listed, or you try to connect to the Library and it does not fully connect. Go to Preferences, under the Sharing pane make sure you have the "Look for shared libraries" box checked. If that does not work, go to System Preferences and depending on your version of OS X, look for Firewall, or go to Security and choose the Firewall pane. Click on "Set access for specific services and applications". Click on the plus sign and manually add iTunes.
Now, if you wish to have sharing from your Mac to Windows XP machines or a Windows XP machine to Macs is not as easy to do if you have a Firewall enabled. It involves using an interface that is not very intuitive.
After you have enabled Sharing in iTunes as written above, you want to do the following on both the Windows machine hosting or receiving the iTunes Library:
Click on the Start button, then go to Control Panel. Click on Network Connections.
Double-click on the Ethernet or Wireless connection that you use. Click on Properties, then click the Advanced tab, then click Settings, and click Advanced, and then select your internet connection (wifi or ethernet) and finally choose Add. Type "iTunes Sharing" for the description. Both Internal and External Ports should be 3689. Select TCP. Click OK to close.
Now Add one more Service. Name the description "Multicast DNS". Both the Internal and External Ports should be 5353. Select UDP. Now click OK for the Service, then click OK again and exit.
Now your iTunes Library will show up on the left side of your iTunes in Windows XP or the Mac on the local network.