Something called XP antivirus 2012 unregistered wont leave me alone!?

How do i Get rid of it? it wont let me use the internet because it is a "threat". and i cant play games because it interrupts me every 5 minutes (like clockwork) with like 4 windows explaining that i have threats. i cant do anything about it though, as i need to buy it or register it. i don't have a code, so wtf? also i have never seen it before. maybe it was a result oshuttingng down my computer and checking yes to "Shut down and update". all i want is to get rid of this stupid thing.