Hello Owen,
I think the forum software stores that option in a cookie in your Web browser, along with your username and password. You mentioned that you reinstalled Windows on your PC recently, so I'd guess that your problem with auto-login is related to a change that occurred in that process.
Try clearing your browser's cache, including all cookies, and all saved usernames/passwords, and all saved data. Then restart the browser, then try logging into the forum and ticking that "Log me on automatically ..." option (being extra careful to make sure you enter the correct username/password when doing so, so that it doesn't save an incorrect one). Then close the browser, reopen it, go to the forum, and see whether it logs you in automatically.
If it doesn't, check in your browser's settings that the browser isn't automatically clearing cookies when it closes (or for each new site/page). Some recent versions of browsers clear cookies by default (as a privacy/anti-tracking measure), such as Firefox. If the browser is clearing the cookies automatically then the forum's auto-login option won't work, because the browser will have deleted its cookie.