Even if you install windows 7 on another partition the old data of previous operating system still present on your hard drive which becomes undeletable or inaccessible from Windows 7.
Usually this problem of undeletable files and folder is because of Access control list of these object is associated with SID of old operating system user accounts and these users are not present in your windows 7, effectively data is owned by nobody or non-existent user.
In order to access the files and folder for manipulation, modification or deletion, we will need to take control of the files under Windows 7. Here are two command line syntaxes to instantly take ownership of such files,
For Files:
takeown /f file_name /d y
icacls file_name /grant administrators:F
For Directories (will perform action recursively):
takeown /f directory_name /r /d y
icacls directory_name /grant administrators:F /t