The password for your DSL will remain the same.
To protect your laptop from being used by others, password-protect the user account. Password - Create for a User Account
Now, for the anti-virus. Download Avast from the internet onto your desktop. Plug the pen drive into one of your desktop USB ports, and wait until Windows installs the drivers
Then go to My Computer, and open your drive. Drag the Avast! installer into the drive.
In the system try, at the bottom-right, you will see an icon labeled Safely Remove Hardware
. Click it, and eject your pen drive.
Unplug the drive from your desktop, and plug it into your laptop. Open the drive up in My Computer, and run the installer you put on there.
Let us know if you need more help.