I'm guessing the default browser is IE7 (maybe even IE6?) for Windows XP.

The stats that matter are what browser they are using, which is less than 1% for IE7, and possibly near zero depending on your target market.

Edit: I see, it's a Windows XP issue rather than an IE issue.

http://serverfault.com/questions/389806/redirect-to-ssl-only...

That doesn't matter at all. XP doesn't support SNI, therefore every application which uses XP's SSPI libraries doesn't either. So IE6-8 and Safari on XP all don't support SNI.

Chrome on XP does support SNI but that is because they don't use XP's SSPI library for SSL connections (they use Mozilla's library NSS).

IE on XP does not support SNI, regardless of version (6, 7 or 8).

On a mixture of around 100 consumer facing UK sites (think small local businesses) that I have access to, Windows XP made up 3% of sessions in the last month, that's out of over 100k sessions according to Analytics.