What is your OS version? Always include that information when you post a question.
You have to find com.apple.finder.plist
Then you drag it to your Desktop and Restart
It is in your Home/Library/Preferences folder (Home is your User Name; you know it, we don't.)
If you have Lion, hold down the Option key and click on the Go menu to get into the correct Library folder. There are at least three of them, and only this one will have any effect on the behavior of Finder in your User Account.