Many people have been getting IM messages lately from random avatars that try to get you to click a link. Messages promising you free lindens or other things. What ever these messages say, do not click on the link! This is a Scam!
If you did click the link, or even worse, entered your account information or downloaded something you immediately want to action if you do not want to lose your account!
If you entered your account information (incl. password): Understand that “hackers” now have your information. Immediately change your password and report this to LL. If you are using the same password for any other account (ex. email) change these passwords too! Also restart your router so that your IP address changes.
If you downloaded something: Turn off your Internet now (just turn your router off)! Remove the software, scan for viruses and/or ask someone you know that has experience to check your computer. The software that you downloaded most likely contains a Trojan. Consider that all your private information (that you have on your computer) is now in the hands of some “hackers”. Also make sure to change your passwords (using a other computer or your phone, until your computer is checked and free from Viruses/Trojans).