You need to take ownership of the folder and it's contents, and then delete it. If you right click on the user folder and go to the security settings, and then click advanced, you should see an 'Owner' tab. Selecting that will display the current owner as the GUID (random ID string) of the deleted user, and other accounts which can take ownership of the folder. Click the "Edit" button and select your user account, check 'Repalce owner on subcontainers and objects", and then click apply. Once it is done, you should be able to delete the folder normally.
You could also try launching windows explorer as an administrator (Task Manager -> Show All Processes -> File, New Task (It should warn that the process will be created with administrative privileges), and then run
explorer.exe. This should open a new explorer window running with full privileges. Be very, very careful. Try deleting the user folder with the Admin'd Explorer window. Close it immediately afterwards - it will let you mess up your system very quickly, and you don't want to accidentally use it.