As it turns out, this is a very important question. Without Microsoft’s support, next to your own behavior, your anti-malware tools are your most important line of defense.
But there is a more nuanced issue going on here. Let’s talk about that.
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What will happen with XP
There’s no blanket answer to your question. For example, it’s unclear if Microsoft will continue to update their Microsoft Security Essentials for XP. (Update: Microsoft has announced that they will keep providing malware database updates into 2015.) I suspect that they probably will until a technical issue appears that would make it difficult for them to continue to do so. At that point, Microsoft will probably drop support.
As for other programs? You’ll have to check with the individual anti-malware tool manufacturers.
They will all have different answers. In the end, I believe they will follow the strategy of Microsoft Security Essentials and support XP for “a while”.
That nuanced issue
You need to remember that just because other manufacturers aren’t updating their programs doesn’t mean that you necessarily aren’t protected.
The anti-malware program is one thing. That means the latest version of their anti-malware program may not be installed or work on Windows XP.
The other piece of the anti-malware puzzle is the database of current threats. My hope is that the Windows XP version of your anti-malware tool continues to share the same database that the most current versions of the program do. That way, you may not have new software, but the last database used by the version you do have still protects your XP from the latest in-the-wild threats.
Again, we just don’t know; this is going to vary from one program to another. While I expect XP support to continue in many anti-malware tools for some time, I can’t tell you right now which ones will or for how long they will do so. Ultimately, only time will tell.