If it's more than 5 years old, a good choice is to make it into a downloader. Bare minimum OS needs and just the software needed to get online and do the downloading. Needs a reliable burner likely. But good dvd burners are cheap as water.
Needs a large hard drive, but those are cheap too.
Doesn't need fancy video or audio cards, you are not playing on it.
Likely needs minimum ram, as you ain't playing on it.
Doesn't need anything other than a junky mouse or keyboard.
And you can use a crummy monitor too as you only need the basics.
The benefit is if it's just a downloader you don't need to fret over performance. You can have firewalls running and antivirus on full bore because it's not going to mess with gaming.
As long as you have all the key pieces already, it will be fine as a downloader. Because downloading is about your service, not the quality of the computer.
You can also call it a homework computer. And then all you need is a good word processor and internet access. And all of the above is still fine.
Now if you are in need of advice for where to get any needed missing pieces, that's different.
If you were Canadian, I would send you here.
http://www.canadacomputers.com/
That's the retailer of choice (my opinion).
