XAMPP and Drupal again (sorry)

Unfortunately, all I can really do is guess, never having run into this, myself. It'd make sense if it was rejecting the username and password for the database (since this would mean either the ones being used were wrong or the settings file was wrong), but clicking on the "create first user" link takes one to a form for a username and email address and then to the profile created for the username provided at which time you can change the password for that account. From what you're saying, you've already created the account and set the password and simply can't log back in for some reason.

If you still have a2t installed, it'd be worth checking the apache log and the mysql log on your machine and see if there are any clues there.

Failing that, you might uninstall all three and then double-check to make sure the actual directories they were installed to were deleted, deleting them yourself if not, double-check to make sure the my.ini and php.ini files in the Windows directory were removed and delete them too otherwise, then reinstall a2t (and, of course, back up anything you need to save that might be in those directories before actually trashing them). It seems like it might be something in your installation since I've gone through various installations of a2t, Xampp and so on over the last few years and not seen anything similar (a2t's installer is pretty brute force and generally overwrites or deletes anything that might conflict with a different apache configuration when it's installed or removed).

I dug around on google and found similar issues for different CMS' with XAMPP (wordpress, etc.) but there doesn't seem to be a common cause among them. You might check to make sure Windows' firewall or whatever you use is not blocking it and check to make sure that, once all the lamp stacks are uninstalled, that if you hit crtl alt delete you don't see mysql or apache services still running in the background (httpd.exe, mysqld.exe).

If it's something like a corrupt registration of the mysql files, they can be troubleshot as such, but you should also have errors or warnings to that effect in your logs, too.

Did you remember to assign a db user for the databases you created and allow them full priveleges on the databases? And also check to make sure you are using the correct mysql host.