The "RandomLetters" are probably a temporary computer name under which the accounts were created (and using the other accounts of the same name as templates). I'm guessing that either a virus or SpyWare was attempting to set up a backdoor, or an update ran amock and this extraneous account creation was an unintended side-effect.
When an account, say "Administrator" for this example, already exists on a computer, and then you login to a network account by the same name, a local account called "Administrator.NETWORK_NAME" gets created to avoid a conflict with the local account that doesn't have that trailing computer name.
I've also noticed that accounts are sometimes created with this network or computer name added after a account of the same name was previous deleted by the data files remained. Obviously this is caused by Windows re-creating the account in a manner that doesn't conflict with another account of the same name (especially since some files in the user's "Documents and Settings" profile directories can't be deleted).