A new worm has affected Goo.gl links on Twitter. You know, the simple URL shortener by Google that integrates with your Google Account and lets you track clicks.
The worm is using others' Twitter accounts to spread links to an 'anti-virus' called "Security Shield". It's in fact malware, as reported by Sophos and Kaspersky.
If you find that your account has been compromised, you should immediately change your password. Also scan your computer for malware. It'll just do you good.