buying and installing Windows 7

We have a spare HP PC that came with XP but a year ago Microsoft was selling W8 for a special price and I bought it and my friend installed it. Now after using it, I really don't like it and I would like to ask if it is economical to buy a licensed copy of Windows 7 (which is what our main pc and laptops have). There a lot of offers for "builder's" versions around. Would that work in this case?

Thanks, John