Microsoft has announced a security bulletin outlining a vulnerability in the way Windows handles animated cursors. MS is advising people to not click on any unknown emails, or any unsafe or questionable websites:

Microsoft is investigating new public reports of attacks exploiting a vulnerability in the way Microsoft Windows handles animated cursor (.ani) files. In order for this attack to be carried out, a user must either visit a Web site that contains a Web page that is used to exploit the vulnerability or view a specially crafted e-mail message or email attachment sent to them by an attacker.

Source: Microsoft