What reason did the sales assistant give for it not working, technical or legal?
If it works depends on a number of factors.
First the laptop needs to be running Windows, is that on your laptop?
Also it depends how many licenses your version of Office allows. Some only alow you to install office on one PC, and if you have already done that you need to remove it off that one and put it on this one.
However you mention £100 so my guess is you have bought MS Office Home and Student and this allows you to put it on 3 different computers. So if you have not already installed it on 3 you should be able to go ahead and install it.
p.s. if it is a new laptop with Windows 7 on it then it may well have MS Office Starter which is a free version of Word and Excel (which have slighlty less function that the normal Word and Excel but are fine for most people). Even better they are not time limited so will work for as long as you have the laptop.