Are you logged on as an Administrator so you can work with your web server's backend like services etc.?
Have you disabled UAC if not here's how: To disable UAC go to Start>Control Panel>User Accounts there you will find an option Turn User Account Control on or off just click on it and uncheck User Account Control to help protect your computer click OK.
After this reboot, login as an Admin and try and do the things you got access denied again.
Open http://localhost or http://localhost/xampp/index.php for the XAMPP Welcome Demo Page and look for and click on the Security menu item to get to the link to set root super user password for MySQL and phpMyAdmin.
This is also mention in the readme_en.txt file under Security A Must Read!