A very large security vulnerability was found this weekend which affects all versions of Internet Explorer on all versions of Microsoft Windows. The attack is delivered via Internet Explorer and Adobe Flash. If you visit a website designed to exploit this security hole, your computer will be compromised. Hackers can execute any code they want on your computer. As of today, April 28, 2014 there is no fix for it from Microsoft. You can see Microsoft's security advisory here https://technet.microsoft.com/en-US/library/security/2963983.