If you know of a 2 letter .com domain name that is still available, and the "letters" are valid within the RFC for domain names, you should be able to to register it at any registrar.
Valid characters include numbers (0-9), letters (a-z) and the hyphen (-). Domain names may not begin with a hyphen.
Which registrar were you using, and what error did you get when you tried to register the domain?
If you are afraid of losing the domain name by talking about this, email me, PM me, or post a note in our helpdesk forum on my site, and I'll be happy to research this for you with full assurances that I will not steal your domain name if it's registerable.
As timechange said, all 676 2 letter .com domains are registered (and very valuable - especially if they can be used as a pseudo cctld).
Also, all 17,576 3 letter .com domains are also registered.
1 letter domains are subject to reservation by IANA, and the few that are in usage cannot be registered by a third party even if deleted (as was the case for e.com a few years back which was briefly held by a new registrant before IANA transferred it away and reserved it).