Well, Microsoft won't be patching it anymore, so any future vulnerabilities that are found will be easily exploited and it's unlikely anyone will be fixing them. Some people might take it upon themselves to put out patches but they won't be official in any way.
If you have the computer behind a firewall or NAT device (like a router) and you're careful about what you install you should
be okay, but honestly, unless you have an explicit reason to stick with XP (such as a critical program that doesn't work in higher versions) then you should upgrade or switch to Linux or something. Web sites may even stop supporting computers running XP for security reasons.