What is the Heartbleed bug?
It is a vulnerability in the module that provides security to internet communication used by many web servers. It is NOT a problem for Windows computers or servers. It has NOTHING to do with the end of support for Windows XP.
More details can be found on the official Heartbleed website of the security researchers who announced the bug and on Wikipedia.
Is my website affected?
You can run a test on your website or on websites you access by SSL Labs’ SLL Server Test. You’re hoping for a “This server is not vulnerable to the Heartbleed attack. (Experimental)” result.
What can I do?
If you administer a web server you should already know the answer to this. For most people it is worth considering changing your password for websites that have been impacted. LastPass (which we highly recommend) offers a Security Check which advises which accounts have been impacted.