If you're using some version of Windows Server, it's pretty easy to set up your own Windows Server Update Service (WSUS). Visit http://technet.microsoft.com/en-us/wsus/ to download, install and configure WSUS.
Now, in order to have your PCs reference your WSUS rather than Windows Update, you need to change a couple of policy objects. Since I'm using a domain for my home network, I use the Group Policy Object Editor once and all the PCs pick it up. I believe the same changes will work by applying them to a PC's Local Policies.
Open the Policy Object Editor and navigate to Administrative Templates / Windows Components / Windows Update.
The pane to the right will show the applicable settings for Windows Update.
Now you need to open the policy for Specify intranet Microsoft update service location Properties and provide the URL to your WSUS machine. You may have to check in IIS Manager to determine the correct port.
Your domain-connected PCs will pick up the change on their next policy load. You'll know that the client PCs are using your home WSUS when they appear in the Computers list of the WSUS admin application (Administrative Tools / Microsoft Windows Server Update Services 3.0)
