I bought a Dell computer a few years back. It has MS office for small business. I recently bought a Compaq laptop which had a 60 day trial of MS Office. It has expired. My question is can I use the backup copy MS Office CD from the Dell computer and load it to the laptop to avoid having to purchase a new copy? Thanks.

If it is "XP" then you need to register each installation with Microsquash. Older software can be shared, but it is not technically legal.

If you have the CD for the small business version, then just ininstall the Trial version from the compaq, and then install the small business version. Nothing illegal about that at all.
It gets legally sticky though if you are trying to use one copy on multiple machines- In general the liscense agreements are for an indivudual use. You could install it on another computer, as long as you were not still using the old installation on the other computer. That is the legalese of it- although with the older stuff, there really wasn't any checking. Now, MS makes you register and activate when you install it- so if its already been activated more than a couple times they will lock you out- but that is the current versions of office.

The software has a way of checking for the numbers on the motherboard. If you purchase a copy of MS office, you can install it on three computers. If the copy came on a computer it will only work with that computer.

The software has a way of checking for the numbers on the motherboard. If you purchase a copy of MS office, you can install it on three computers. If the copy came on a computer it will only work with that computer.
Correct.

Correct.
I love hearing that.