I've been using my computer on a network where there is a Software Update Server (SUS) available, but since my computer is not managed by Tech Support, I've been unable to use it. I recently found the secret location of the preference that Workgroup Manager saves the SUS location in; it's in /Library -> Managed Preferences -> short_username -> com.apple.SoftwareUpdate.plist.
Copy the following code into a plain text document and save it as com.apple.SoftwareUpdate.plist in the above mentioned location. Make sure to edit the string of CatalogURL to your specific SUS URL:
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple Computer//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
<key>CatalogURL</key>
<string>http://PUT THE URL OF YOUR SUS HERE/</string>
<key>WebKitDefaultFontSize</key>
<integer>11</integer>
<key>WebKitStandardFont</key>
<string>Lucida Grande</string>
</dict>
</plist>
Mac OS X Hints
http://hints.macworld.com/article.php?story=20060516093639166