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