Several users are reporting a server problem with Software Update. Why not wait and try again later today or tomorrow to see if the problem is resolved.
If the problem is on your computer then give this a try:
Fix for Software Update
Delete the following files:
- /Home/Library/Preferences/ByHost/com.apple.SoftwareUpdate.xxx.plist where "xxx" is a number.
- /Home/Library/Caches/com.apple.SoftwareUpdate/ (delete entire folder)