Microsoft likes to hang onto its software just about to antiquity, but there are many options under $100. Do you happen to know if the PC is 32-bit or 64-bit? Most everything now these days is 64-bit. So here's a link to a 64-bit Window 7 professional.
Comes to $69 even with free shipping. Though if you're capable of burning an image to a DVD. You could buy a license off limits on Amazon for almost nothing and when you get the key code. You can download an image of Windows 7 off of Microsoft website for free if you enter the license code. Then you downloaded and have the image to put on the DVD and they got pretty simple.
for $47.77 or
Microsoft's download page
The gentleman above me beat me while I was typing, but here's a few more options.
I used through repair electronics out of my home until I lost the use of my hands. If you're interested in any of these options or the one above and need any help I'd be glad to lend it to you!
Edited by The_Atheistic_One, 24 August 2016 - 08:11 AM.