While it may not be apparent to bank account holders, about 95 percent of ATMs in the U.S. run Windows XP.
"Banks are now spending a lot of money to refurbish their ATMs. They're even paying Microsoft to extend their protection just for a few months just while they get their act together and build new ATM machines with different software," Carey said.
Read More End of Windows XP leaves many companies vulnerable
Individual Windows XP users should also upgrade to a new operating system, Carey said.
"It's not that a hacker is going to figure out how to crack it, but now there's nothing to stop them," Carey said. "You have to upgrade, because right now if you don't, you have a big target painted on your computer. For a while now, we've know that this was the day that Windows will no longer send out a patch if there was a vulnerability or a security problem that's discovered now," Carey said.
—By CNBC's Althea Chang