Did you try logging in using the name of the folder that contains your mail data C:\Pmail\mail\<username>\ 

One thing I would be tempted to try is to put Pegasus Mail into multi-user mode.

Restore a copy of your archived data.
Delete pmail.cfg

Run Pegasus Mail - it should ask you which mode you wish to run it in - choose network - it will then ask you for the location of the user data. Point it at the copy you restored. When you do, it will list the usernames of the account(s) in that folder.

Otherwise, it sounds as though you might have a corrupt archive. Even so, you may be able to retrieve the data.

EDIT

It just occurred to me that your backups may not consist of the entire pmail folder structure. Before you do this can you confirm exactly what you are archiving - which folders?

I have archived my old emails as PMAIL files on drive C, labeled according to date, and when I access them I rename the file to "PMAIL" and then run the application. This works beautifully except for one file which won't open. When I run the app it requests a username, and will not accept any username I've tried. (admin, my name, new mail, etc.) I am not particularly tech savvy and this is not critical as I need to get into that file. Is there any way around the username requirement? I don't understand why it only asks for a username with only one file.

Can you explain what you are doing here? When you say old emails, are you referring to cnm files, or are you referring to folders - pmm+pmi files? Which file will not open and what, exactly, are you renaming? When you say Pegasus Mail requests a username and will not accept the login name you usually supply - this implies you have replaced the pmail.usr file with another. If so, open the .usr file in a text editor and the username will be listed.

These archives sound like entire copies of the \Pmail directory.  If so, I suspect the problem is that a configuration file in the problem archive has been damaged (possible pmail.usr in the route of \Mail (if it exists) or pmail.ini in the mailbox directory).  You might try renaming them then copying in ones from an archive most closely dated to the bad one  This may not work if there were dramatic configuration changed between the two archives but it's worth a try and is easy to reverse.

If these are entire backups have you taken steps to insure that when you run PMail from one of them that you have disabled any downloading of messages to prevent new messages from being downloaded into an archive when in use?

Yes, they are copies of the entire Pmail file. The .usr file has "Admin;Mail Administrator" just like the others. I tried copying the PMAIL.INI fle from one of the others that opens without the username prompt (with high hopes) but got the same prompt when I ran the file I am trying to open. I put in "Admin" and got the same 'does not exist on this system' message. 

The program is set to retrieve manually. 

I don't know what is in that file, it may be the oldest emails that ran on a previous version of Pegasus. Would that make a difference? 

It's a way of archiving - remember I said I was not tech-savvy - where I rename the entire PMAIL file according to the time period of the emails. That way there aren't so many to open at a time. The .usr file has 'Admin' as the username, and I've tried that many times but get 'doesn't exist on the system'. The other files open without the username prompt.

There will be a folder within your PMail directory which will either exist off .\PMail or as a subfolder off .\PMail\Mail\

The name of the folder will be the name of the account that is used to login to that mail folder. I don't understand how, if these are complete backups, the .usr file can not reflect the specified username. Another question - are all these backups for the same mail account or did/do you use multiple mail accounts?

Pegasus Mail is backwards compatible and quite bomb-proof with regards to preserving your data. Older archives should not make a difference so long as you are restoring just the content of the mail account folders.

I did not have multiple mail accounts. 

The current version of Pegasus brings up the other archived files fine, it's just this one where it asks for a username and then doesn't accept any.