You should be able to use your account on any computer, even on one owned by another AH player. No need to create a shade.
First, can you download the installation package on your new computer? You don't need any username or password for that.
If the installing goes as supposed, the first time where you need those is when you try to log in the main arena. Offline should work without them.
If it's the online login screen where you get the error messages, are you 100% positive that both your username and password are currently valid? I mean, have you ever closed your account and created a new one later? If so, you may be trying with your old account information. The BBS login info won't work, either.
Here's the best part: If you've been able to automatically log in on your previous computer, your username and password are saved in the Settings folder as "login.cfg" which you can open with Notepad.
<edit> Skuzzy the Lightning Finger beat me again.
