Just check to see how much of an install they do. Some places just run the Windows disc, install the drivers and leave you with several hours of updates to download and install at home.
If the old drive is still working but is either too small or has bad sectors then copying across his "My Documents" folder with pictures and files in it is fairly straightforward.
Installing drivers for printers or scanners that you have at home is usually your job, as the shop won't know what you have.
TheComputerLab is about right, £60 for the drive should get you a decent capacity one plus £40 for the install plus all the updates, drivers and copying pictures etc from a working drive.