If you use Windows XP and have Adobe Reader version 7 – 8.1 you need to patch it. Use Adobe’s built in patching system to update it.
The vulnerability was discovered last month and there are already exploits that can allow attackers to take over your computer. Interestingly one of the known vulnerabilities was created by a Russian online crime syndicate called RBN (Russian Business Network). They’re responsible for a large amount of online crime.
A bulletin from Adobe is here, for reference.