The Windows 7 Welcome screen is a security feature to prevent unauthorized access to your computer. However, for most people, it is an inconvenience rather than a security feature. Well, fear not. This guide will help you modify this security feature so you can log into Windows faster.
Step 1: Search for netplwiz in the Windows search box and hit the Enter key.
Step 2: Uncheck the box next to "Users must enter a username and password to use this computer" and click OK.
Step 3: Enter the username you’d like to automatically log on and the password for that account, then click OK.
That’s it. Now your Windows 7 computer will boot into the Windows desktop automatically. Aside from being convenient, this is a good way to record boot times, if you’re into that sort of thing.
Just remember, once you do this, anyone who comes across your computer can access your files, make changes to your system, or install malware with ease. Be warned!
Source: cNet Howto