If you use Avast for your antivirus program, then you need to update it to fix a serious security problem. Reported by US Cert:
Avast! antivirus contains a buffer overflow vulnerability. This vulnerability may allow an attacker to execute code a vulnerable system.
In short, the bad buys can take control of your computer if Avast isn't updated to 4.7.700. The automatic updater should alert you. If not, you can download it from the Avast download page. Most people probably have the free home version, which is the 2nd link on the download page.