Can an Sys Admin prevent/stop SAM extraction?
As long as you can get in as Administrator, you are basically vulnerable. Microsoft has gradually increased its security for the SAM files and the hashes, but as things like L0phtCrack are quickly improved and Microsoft insists on backward compatibility with LAN Manager-style logins, things will be vulnerable. In fact, the latest L0phtCrack can take input from stored sniffer traces to use as the source for its password cracking. So for you sys admins out there, keep absolutely current of Service Packs and Hot Fixes.