After april 8th 2014 Microsoft will no longer support Windows XP. Microsoft will publish no new security pathches for vulnerabilities and Security Essentials will only provide new malware signatures for a short while. Windows XP will be orphaned. By Microsoft. But many consumers and companies still use XP systems. They will be at risk. Because XP will not be orphaned by criminals. Vulnerabilities in newer Windows versions will be patched, but since all Windows versions since NT are related, chances are that a vulnerability in a more recent version is a vulnerability in XP too. While it will be fixed in Vista, 7 and 8 and will no longer be a vulnerability in those systems, such a vulnerability will still exist in XP and will be exploited by criminals. From april 8th this will be bad luck for XP users. At least for consumers. Most consumers will run a fully patched XP system, since Microsoft advises you to. So, for consumers to move to a more recent operating system is the only way to stay secure. But so far consumer pc's are not a big problem.
But there is an even bigger risk and it is not even a new risk: Enterprises and other organisations running XP systems. Those organisations have a serious problem. If they still run XP, then they have a change management problem. Commercial support for XP ended long time ago, meaning that they gambled that they would not need any support.
And we may have a big problem with that. In professional surroundings most workstations are managed centrally, from an update server. Patches are not downloaded directly from the Microsoft servers, but these organisation distribute patches in releases after they tested the patches. And such a process may take a while. Many companies roll-out new releases once in a while, two or three month intervals are common. Changing more frequent is often seen as a risk, because these (unsupported) systems better not be touched. Never change a winning team...
Will these systems be at risk after april 8th? Sure. Are they at risk now? You bet. If a company is still running XP at this moment, chances are that older patches have not been applied either. Stay away from organisations running XP, the problem is bigger than you imagined.