If you use an email program (client), expect it to get infected and email viruses to all your contacts. If you use your XP system for online banking, expect your account to be hacked and all your money taken. And Windows 7 is also no longer patched for free as of January this year.
Peronally, I have some legacy XP systems for running old third party software, but wouldn't even think about using an XP system for my main internet-facing system. It's even hard these days to find a realtime antivirus for XP that still has definitions updated and doesn't have a zillion nag popups. By now it has many security holes which no security software (antivirus, firewall) will protect you against. XP hasn't been patched by Microsoft for many years.
