If your computer was setup to logon to your account
automatically before you installed the .NET Framework, and
you'd like to restore that feature, try the following
procedure:

Go to Start -> Run.
Copy and paste the following into the Open box:

control userpasswords2

Click OK.
Uncheck "Users must enter a user name and password to use this
computer".
In the box that pops up, replace Administrator in the "User
Name" box with your user account. Enter your password twice.
Note: You can leave the password box blank if your account does
not have a password.
Click OK twice and reboot to see if you get the desired
results.

Here's a web site with some additional info:

Skip the Welcome Screen and Logon Automatically
http://mvps.org/marksxp/WindowsXP/welskip.php

Good luck

Nepatsfan