The hardest task for any 'novice' computer buyer is knowing what they need it for now and in the future.
Do they need it to work @ home ?
Will the children wish to play games on it ?
How many £'s have you got spare ?
Do you need the latest spec all singing all dancing PC ?
It may come with Windows XP Home but do you need all the other Microsoft software or will Open Office for example do ?
Where is it going to be living, bedroom, children's room, study ?
Breakdown cover, one year on site plus two years return to base or three years on site etc. ?
The list goes on and on
Great idea but you can see it would be difficult to cover everyone's needs, even if they knew what they were.
My advice for what's it's worth would be to pick a target price and a list of requirements and stick to it, make the sales staff work for their cut, it's your hard earned money after all.