Depends if you have Home or Pro. If Home: start the machine in safe-mode (hit F5 or F8 during boot) & then rigth-click the folder & choose the security-tab. There should be a button "advanced" & via that you have to take the ownership of the folder. You maybe have to change a folder setting before you can see the security-tab: I think the english version for that is "Enable simple filesharing". If that is checked & no security-tab, uncheck it or vice versa.
If Pro: exactly the same as above but you don't have to go to Safe-mode.