2 causes are that you have nothing to unzip a 7z file with, or that you tried to open up a 7z file before with notepad and now the extension is associated with it.

To fix, make sure you have something that can unzip a 7z file, like 7zip or winRAR.  Right click on the file and mouse over open with on the menu that comes up, then select choose default program in the side menu.  click on browse, navigate to whichever program you've got and make sure the always use selected program box is checked, select, the program, click okay and you're off and running.

Another option to unassociate the file type is to download a program called Unassociate from here http://www.winhelponline.com/downloadattachment.php?aId=c060d239de9af876a20a6374973bb60a&articleId=231  which is a utility for windows vista/7 to unassociate file types.  Once you undo the .7z association use the real program you're supposed to use to unzip it and it should reassociate the extension correctly.