When I tried the link leocg suggested, I got a "404 — Not Found" message. However, I located this one for version 36.0.2130.65:
http://www.opera.com/download/guide/?os=windows&ver=36.0.2130.65&local=y
It installed OK, but it looks like this version won't allow extensions and add-ons. When looking for some of them, it says I need to upgrade to the latest version to use them.
I had not used Opera on this computer before but I have used Internet Explorer, Google Chrome and Firefox. Of those, Chrome worked best, but it is no longer supported for XP. I have had no problems lately with Avast.