A little known secret about Microsoft's Internet Explorer Web browser is the long-standing feature that lets Web sites silently read data stored in the Windows "clipboard" -- the storage space that serves as a semi-temporary repository for any text the user has recently cut-and-pasted or copied in virtually any Windows program.(Source
Apparently, Microsoft has finally changed that feature with IE 7. The newest version of the browser throws up a prompt asking users whether they really want to share the contents of their clipboard (should they stumble upon a site that tries to filch it).
Or, you could use Firefox or Opera, which never shared the clipboard file with websites.