Ron says that Microsoft Security Essentials is no longer being supported on XP. Leo says that Microsoft will continue to support MSE, just not update the operating system itself, or the MSE program. It'll be able to continue to scan for viruses with new definitions, so that's not to worry. Steps to protect yourself since Microsoft has stopped supporting XP:
1. Stop using XP as an administrator. Use it as a limited user instead. Add an account as an administrator and then demote your existing account to limited user. This will stop over 90% of all the exploits out there.
2. Stop using Internet Explorer. Go with Google Chrome. It's free and far more secure.
3. Don't click on links in email.
4. Only get your software from original vendors.
5. Keep your anti virus software up to date.
6. Stop using Java
7. Use a password vault like LastPass
8. Turn on second factor authentication
You do these things, you'll be fine moving forward. And you can still use Security Essentials. But remember that this stuff won't help if you don't protect yourself through your own behavior.