Your computer won’t stop working in April. It will keep chugging along doing what it’s doing now. The only thing that happens in April is that Microsoft will officially stop making patches for it.
You will need to keep your anti-virus up to date, and if you use this for web browsing you will want to keep your browser up to date. That will stop most of the baddies from harming your computer. If someone discovers a security flaw in XP during that time you will be vulnerable to it. In the past, Microsoft has patched obsolete operating systems when there was a major security exploit going around, but that’s no guarantee that they will continue to do so, and they don’t patch the little security holes that are found that aren’t causing users big problems.
Keep the machine behind a firewall and don’t worry about it.