Helpdesk positions are the greatest if you want to learn ANYTHING. Especially if you need to cover an evening shift, you've got lots of gear and lots of time.
You have the entire computer inventory and assets of the company at your disposal - buy a book and learn something that someone is hiring for, then go interview.
Linux is a good choice, but so is SQL, Oracle, Networking and Security, or Virtualization. Essentially just pick something (anything) and become an expert.
Learn anything that isn't Windows and AD, because the perception is that anyone coming out of college or highschool knows that stuff - this isn't true by the way, but you're not going to convince the person who's interviewing you of that.
Alternatively, if you can swing a student loan, go back to school for 2-3 years and get a masters, there will never be a better time than right now - this is always true at a personal level, no matter what the economic situation is.