Well here is the downside. XP is no longer supported by Microsoft. This is for both bug fixes and security fixes. Bug fixes are one thing and I wouldn't worry too much about that but the vulnerability/security fixes are another. If a bad guy finds a way in to XP, and this happens quite a bit, Microsoft will not be patching the hole. There are still a lot of places that use XP but are closed off from the broader internet to keep these types of intrusion at a minimum.

If you are using XP and want to continue to use XP then make sure to keep your virus and malware programs up to date. They would continue to be updated and supported by the developers (Norton, McAffee, Panda, etc). Or you could sell your machine to Ralph from Vegas. He'd love to go back to XP.