does it work if you try logging in as a different user? if yes, then it's probably a corrupt plist file in your home/library/preferences or the cache in home/library/caches.
if it doesn't work logged in as another user either, then try deleting the cache and/or the plist for software update in the library directory at the root of your drive.
also, maybe d/l the 10.2.6 *combo* update and re-apply that.....