Robin,
Well the op. sys shouldn't matter as long as they are both microsoft, it's whether the data files can be read by the applications on the newer machine (for example do you have MS Word on both machines?.
If your dh fitted a new drive all well and good, but sometimes you need to change internal switches on a machine to accept an additional drive, so make sure you have the documentation for your motherboard/existing drives available (actually it's my dh who does this in our house,so he'd know more).
Answer is yes it can be done, but do you know enough about your kit to do it yourselves?