I am trying to delete a certain folder, but The above message keeps on coming and I can,t delete. It rather suggest that, "Make sure the disk is not full or write-protected and the file is not currently in use".

I have check all that but still the file can't be deleted. What can I do to delete that folder with its contents?

You need to un-install (un-register) the dll before deleting that from system

Reboot the system in safe mode with networking and try to rename to .dll file to like .dat or .bat. befor that make sure u have full access control.

Is the best way to removing. As explain by sathi.pk

before deletting ensure first that u are the user administrator then if got the same message
right click the said folder or file and go to the properties-advance-uncheck encrypt contents to secure data and try to delete it and if it doesnt work probably contents of that file is running in the background try to use safe mode and repeat the same process and it will surely delete the folder... hope this may help