This user account is automatically created when the .NET framework is installed on your PC. Its only purpose is to allow you to run ASP.NET aps on your PC's local host web server.
If you are not doing any ASP.NET web development you can delete it without any problems, and this should return your Logon to normal.
There are other options besides deleting it, check out the following link for an explanation of these:
http://www.mvps.org/marksxp/WindowsXP/aspdot.php