I generally go out of my way to support older versions of Windows, however Windows XP is now so old that it is simply not practical to support it. As its market share is so low, I will be removing stated support for XP from the download page quite soon.
Regarding your specific issue, the limit is actually with the mIRC website, which uses the intermediate level of TLS support recommended by Mozilla. This includes a range of protocols which provide backward compatibility to some degree, including TLS 1.0, but excludes the most insecure protocols. In this case, the issue is probably that Windows XP does not support AES.
I will be changing this in the next version so that mIRC does not use a secure HTTP connection under XP.
