I have used other programs that detect more clutter on My Hard Drive like MCAFFE Quick Clean its capable of freeing up Hole Gigs, so i reckon CCleaner should be able too.
The Registry Cleaner should be able to clean more i say.
More cleaning = more risk of sweeping anything important for the system
Cleaning up gigabytes ? That is impossible. Unless, of course, it's deleting System Restore Points - which is a bad idea.