Open Windows
Task Manager by right clicking on the
Task Bar and selecting it from the menu.
Click on the
Processes tab and find httpd.exe and mysqld.exe and highlight each one at a time and select to
End Process so they are no longer listed - close
Task Manager.
Now try and delete the xampp folder again.
Here is a post that may help in future for uninstalling XAMPP manually.
viewtopic.php?p=175157#p175157