I strongly recommend building one. You can put a good decent one together for about 400 quid and you'll be able to reuse your mouse, keyboard, and monitor. Probably your CD/DVD drive as well and you'll at least be able to use your old hard drive as extra storage if not as a primary drive. You might even be able to reuse the case. All that will drastically decrease the cost. You won't even have to worry about forking over cash for a new built-in OS just before 7 comes out if you build your own instead of buy.
Find a friend that's really good with this kind of stuff and have him walk you through all of this. It's completely worth the trouble because you'll be able to do this on your own in the future and save a lot of money by just upgrading parts instead of full systems. Not only that, your system will be much much much more reliable than a pre-built one if you choose good quality parts for each component. You'll be able to get it to last at least another five years without upgrading parts if the system works well and the usage is as light as it looks like you're planning it to be. Even much longer with upgrades.
Most importantly more reliable parts means less frantic calls from your family trying to enlist your help to fix their computer. You'll get a lot less random errors that don't make sense so you'll be able to better troubleshoot the problems you do have.
Doing this yourself is also incredibly gratifying and is a tremendous learning experience. I didn't know much about building computers when I built my first one, and nowadays they make putting one together almost self explanatory. You don't even need that many tools. Probably a small screwdriver is all. You could get pretty far by just guessing (which is what I did to build my first one) but it's always nice to have someone that knows what they're doing there to help you. You could even turn it into a family thing. Some time to spend with your father or something.
Sorry if I'm sounding pedantic or trying to push you into doing something you don't want to do, but I would encourage everyone to try this, because it's not as difficult as you would think, and the rewards that come with the cost savings, time savings from less future maintenance, and experience certainly justify the time you spend.