FINAL UPDATE 4th Feb - We've determined that the new LoL client will be able to support TLS 1.2 encryption protocol even on older operating systems. So as long as you're using the new client you'll still be able to purchase RP if using Windows XP or Vista.
UPDATE 21st Jan - This change has been postponed while we conduct further testing. I will update here if anything changes.
**TL;DR: If you want to be able to purchase RP after beginning of Jan 24th you must upgrade to Windows 7 or higher.
On January 24th at approximately 06:00 am AEDT, we will be completing security upgrades to our region to ensure we continue to protect players' game and purchase information.
The short of it is that we're upgrading payment stuff to use TLS 1.2 as an encryption protocol, which is a change that is required to be completed ASAP in order to remain compliant with PCI regulation. The PCI Security Standards Council defines why this change must be made sooner rather than later, [on their site](https://blog.pcisecuritystandards.org/migrating-from-ssl-and-early-tls).
An unfortunate reality is that due to these upgrades, players using a Windows XP or Windows Vista machine to run the League of Legends client will no longer be able to purchase RP in the LoL store. This is because these operating systems do not support the use of TLS 1.2 as an encryption protocol.
Based on our data, there are still a small amount of players that have logged in recently using a XP or Vista computer. Before January 24th, players on XP and Vista will still be able to complete RP purchases but starting on the 24th everyone must be on Windows 7 or higher if they want to purchase RP.
If anyone has any additional questions about this change please reply below and I'll do my best to answer them.