Advice. What brand desktop?

your old comp is a pretty good one, I would just replace the hard drive and while your at it max out the memory, RAM,

the hard drive is the weak link in any computer, it is not a matter of "IF" it will go bad but of "WHEN"

think of the hard drive as a replaceable part like a starter or alternator and just about as easy if not easier to do

and the biggest bang for the buck to ANY computer is increasing the RAM memory
(think of adding RAM as adding headers, cam, intake and 4bbl carb to a stock 2bbl motor)

if your computer came with factory install disks (one or tho CD's with all the original software) you are set, just use them to return your computer to it's as-new configuration

if you dont have those disks anymore or yours never came with them, no problem just buy a new copy of windows, the same as would come with a new computer, and now you will have the disks if or when your new hard drive bites the dust