A lot of computer makers put their own background image on the Windows sign-in screen  Some of them are OK, but in many cases I prefer just the regular Microsoft default.  Here are the steps to change it back to the default:

Warning:  The steps below require editing the Windows registry.  If you mess with the wrong things in the registry, you can cause serious problems with your computer.

1. Launch the registry editor by clicking Start, typing regedit and pressing ENTER.  Click YES if you see a security prompt
2. Browse to the following location:
   HKEY_LOCAL_MACHINE\ SOFTWARE\ Microsoft\ Windows\ CurrentVersion\ Authentication\ LogonUI\ Background
3. Double-click on OEMBackground and change value from 1 to a 0
4. Restart your computer

As an alternative, you can download this reg file and run it to make the changes automatically.  Right-click the link and choose “Save Link As…” to save the file to your computer.