Can i reboot computer using a different computers back up disc?

You should be able to get away with it - I've seen Dell desktops restored using OEM recovery discs from another Dell desktop. It's probably the same for Dell laptops. Tell your cousin to make a note of the product key on the label on the underside of the Inspiron 9400.

usually dell disks come with specific drivers for the specific system. not a major problem though. if the OS is the same should work then just go to support.dell.com and get the right drivers bu using the 5-7 alphanumerical service tag.

on a side note... if there's a virus, it may be easier to just clean the virus instead of reloading the OS...

There are a couple problems here:

1) What you want to do is illegal. After installation (which I doubt will work anyway) the infected computer will not make it through Window's WGA. It will be flagged for piracy. This means that the computer will not get Microsoft updates.

2) Most reinstallation disks If they are not OEM disks are designed to work only on the specific computer it has been made for.

Reboot means to restart your computer (turn it off and turn it back on). You mean restore, reformat, or reload (differnent meanings but all can be taken to mean what you mean). As long as your OS disc is the exact same verion of his OS, then it *should* work. You would still have to manually download the drivers from the Dell website though.