Occasionally I’ve had errors where Software Update refused to download something, telling me that I didn’t have permissions to save the updated file.
In previous systems I think these files went into the /tmp folder, but in Snow Leopard they don’t, and I’ve finally managed to work out where they do go - /Library/Updates.
On my machine, somehow, I’d ended up with a couple of folders inside this one which were owned by another user, and which didn’t have group write permission.
To fix the problem, I did the following in a terminal window:
sudo chown -R
sudo chmod -R a+rwx /Library/Updates
You need to replace
sudo chown -R :admin /Library/Updates
sudo chmod -R g+rwx /Library/Updates