Internet Explorer is a dependency of League of Legends. Many facets of League of Legends, including the store and login screen, are affected by your Internet Explorer settings. This is true even if you have a different default browser. For this reason, many common errors with the PVP.Net portion of League of Legends can be resolved by restoring your Internet Explorer settings to default.
You attempt to purchase RP in the store and your screen looks like this:
Change privacy settings to medium in Internet Explorer (RECOMMENDED)
This is a pictorial guide featuring Internet Explorer 9. If you do not have Internet Explorer 9 you can update to the latest version from Microsoft here.
1. Open Internet Explorer
2. Navigate to Internet Options
3. Navigate to the Privacy Tab
4. Change your slider to Medium
5. Hit Apply
6. It may be necessary to restart League of Legends
Reset your Internet Explorer Settings to default
Occasionally this does not always resolve the problem. The next step is to simply reset your Internet Explorer settings to default. You can find another detailed pictorial guide to doing this here.