Microsoft will keep on supplying signatures for Microsoft Security Essentials, but that doesn't mean it's a good idea to use Microsoft Security Essentials.
In fact, it's a bad idea to continue using Windows XP even now for plenty of reasons, but if you're not letting go of it, then one of the things you should do is to get a better antivirus program. There are plenty of them and nearly all of them have announced that they will support Windows XP for at least another year, usually at least 2 years.
This information comes from a survey of anti-malware vendors performed by AV-Test.org, a leading test lab for such software.
AV-Test continues to perform regular tests on these products on various operating systems including Windows XP. Among the highest-ranking products in the most recent tests were big names like Bitdefender Internet Security 2014 and Kaspersky Internet Security 2014. These companies will continue support for consumers on Windows XP until 2017 and 2018 respectively.
Other products for which support will be extended for at least one more year are listed below. See the AV-Test report for more details on specific vendors.
- Check Point / ZoneAlarm
- G Data
- K7 Computing
- Kaspersky Lab
- Panda Security
- Qihoo 360
- ThreatTrack / Vipre
- Trend Micro
Note Symantec/Norton is not included in this list. According to AV-Test they have not announced their plans yet.