I believe this is a new take on a similar problem I've seen posted a couple of times before. None of the previous fixes seem to work this time, though.
The background is that one of my apps started Software Update upon detecting new updates after launch. After a longish while, the "Checking for new software..." progress bar had reached about halfway, and the update list indicated it had found five updates. At that point however, the progress bar chose to freeze entirely. After a good while and no further progress, I Cancel the update, and attempt to relaunch. Software Update opens, but the aforementioned progress bar freezes immediately. No matter how many hours I leave it there, it will not budge. The computer suffers from no other Internet-related problems or slowness, and other computers in the same network still run through Software Update in a matter of minutes.
I have tried deleting the partially downloaded files in /Library/Updates and restart, to no effect. I have also tried deleting the com.apple.SoftwareUpdate.plist files from both the top and user level /library/preferences folders, likewise to no effect.
Try to move ALL these file in your desktop and restar : youraccount/Library/Preferences/com.apple.SoftwareUpdate.plist youraccount/Library/Preferences/ByHost/com.apple.SoftwareUpdateXXX.plist youraccount/Library/Caches/com.appleSoftwareUpdate HD/Library/Preferences/com.apple.SoftwareUpdate.plist
I agree - deleting the Cache.db file from the directory above did the trick. I also deleted all the files as suggested in the previous post, but without the Cache.db being deleted, it did not work. Once I deleted Cache.db, update started to function. Thank you Dragon (and Bruce).