Opera is now the only major web browser that still supports Windows XP and Vista. If you’re still using either of those operating systems and browse the web, you should definitely stop using Internet Explorer, Firefox, and Chrome, and switch to Opera. Browsing the web is dangerous enough without the added risk of using a browser that has known security vulnerabilities that will never be fixed.
Note that the most recent Opera version that supports Windows XP and Vista is 36. It wasn’t easy to find older versions on the Opera web site, but I eventually found a page that allows you to download any version by platform.
A recent update to Opera 36 addresses security issues that are specific to XP and Vista. The announcement doesn’t mention the actual new version number, but based on my research, it seems to be 36.0.2130.65.
If you’re using Opera on XP or Vista, make sure you install the new version. It should update itself automatically, but you can also download Opera 36.0.2130.65 directly.
I’ve tried to locate release notes for the new version, with no luck. According to the announcement, several security fixes previously applied to later versions were back-ported to Opera 36.