You'll miss out on security updates leaving your pc more volenrable to viruses, trojans and malware, as well as any OS patches for identified flaws in the OS.
If you have a legit OS there is no good reason not to get updates.
Security holes are found in windows all the time. Some are so bad you don't have to visit a site or download anything. All someone has to do is scan a series of IP addresses and check if you're vulnerable. Update windows unless you like bad things on your computer.