Open the registry by clicking on Start --> Run --> Then type Regedit and click OK.
Navigate to the following key.
You will create a new key of DWORD Type specified as "REG_DWORD" below.
Name the new entry the username you would like to Hide.
Then Set the value to 0 which is the default.
Below in the example you can see the ASPNET user is already set to 0 which is why it's not visible when you login to your computer.
This process can be used to either turn on or turn off hidden accounts.