Hi (from a complete Mac newbie),
Had a very similar problem to this - updates stuck on the 'Checking for Updates' spinning icon. Reboot failed to fix problem.
Because I am a newbie, much of what was said above seemed difficult, so I thought I'd just try the one thing I could actually find on my machine, which was to delete:
~/Library/Preferences/com.apple.SoftwareUpdate.pIist.plist
This step alone seemed to fix things for me. Hope this helps someone.
- Patrick