Hey...same thing happened to me yesterday, except after installing some faulty drivers from ATI. Completely corrupted my logon (except Safe Mode worked for me).
If you can't logon to Windows at all, your best bet is to repair Windows (like I did). Check out this guide, created by a Microsoft MVP. However, be aware that this isn't 100% foolproof. If something goes wrong, you could loose all of the data on your computer. (Happened to me a few months ago, so I was a little hesitant this time...fortunately it worked.)
If you'd like to explore other options, give us a shout back with the name of the "tweak program" you used. There may be an easier/less risky method of fixing the problem.
Hope this helps,
P.S. You can change your user name in Windows XP without using a tweak program...just go start->control panel->user accounts->iser name->change name. With this method, you won't risk corrupting Windows logon.
I'm running WinXP Home, SP2, no network. One user, so no log-on required.
I was trying a 'tweak' program, and used an option to change the Windows user name from ''user'' to my own name.
Now when windows boots, I get a log-on icon for ''user'', and clicking on it (or, for that matter, on ''Turn Off Computer'') does nothing.
If I boot up in Safe Mode, I get 2 log-on icons:
''Administrator'' and ''user''.
Neither icon does anything, so I am completely locked out of Windows.