Bernie has a laptop running Windows XP and use it to run software needed for his shop. He is worried because Microsoft has stopped supporting Windows XP and its respecting software. Leo says that XP isn't being updated either and if you have to be online, not having Windows Defender and Security Essentials is the least of your problems. There currently no new malware for Windows XP but the safest course of action would be to stay off line on that machine completely. The best option is to run Windows XP in Virtual Machine on a more recent laptop running newer versions of Windows.
Other things you can do are:
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 to avoid phishing or malware.
4. Only get your software from original vendors.
5. Keep your anti virus software up to date.
6. Stop using Java/Flash.
7. Use a password vault like LastPass.
8. Turn on 2 factor authentication.