cattypuss I presume that you are a beginner as you posted in this area. Buying a motherboard and processor isn't all that differcult what people seem to have problems with is picking the right memory, memory can be very tricky. If this is the first time you have done anything like this then for a first build its simpler to get a motherboard bundle or a barebones bundle like these click hereclick hereclick here as examples theres cheaper and more expensive option if you wish. The big advantage with buyibg this way is usually its slightly cheaper thn buying the individual parts and you know that parts will work together ok. Another company click here as a further example. Please be aware that if you are considering using your old case you might need to upgrade the power supply particularly if you fit a fairly decent graphics card.
You'll probably also have to reactivate Windows, and if your copy is an OEM one, there's a chance Microsoft won't let you do so, though I would have thought a phone call and a little begging might work.