+1 to what Ray said...get Linux.
It is free.
It will take an old, slow computer and bring new life to it.
You don't have to worry a heck of a lot about viruses, they are there, but not much.
You can make it look and act like almost any operating system.
Installs quickly, with very, very few hassles. - This is a biggie. Install Linux and it just recognizes and activates almost everything you have without you having to spend hours pulling out your hair.
Did I mention, It is Free.
Fantastic user support communities.
If you use MS Office, instead use LibreOffice (free)
Do photography work? use darktable and GIMP (free)
Do video work? multiple okay video editors (free) or two high-end, oscar winning systems (Lightworks and DaVinci Resolve) which are expensive.
Watch videos? I can watch anything, from any site and also cast to my chromecast.
Surf the net? Same browsers and better (free)
Do advanced math calculations, equations and formulas? It is all there, free.
Play video games on your computer? Ok, here it isn't so great, but it is moving up and will probably rival other opsys in the next few years.
If you really like the way your Windoze system works, and are concerned about learning a new operating system, not to worry - the Zorin version of Linux was specifically designed to closely emulate Win
http://zorin-os.com/
Finally, you don't have to install Linux to try it. You can run it from a DVD. It will take a long time to start up if doing this way, but once running it at least gives you the idea on how it works.
dave