Hey isianto:
You might have corrupted Software Update caches. This causes a surprising percentage of Software Update problems, and is very easy to correct.
Quit Software Update if it's running.
Navigate to the folder: /Users/username/Library/Caches/
Find these folders: Software Update, com.apple.SoftwareUpdate. Note: you may only find one of these - if so, this is normal.
Send them to the trash.
Restart Software Update.