I clicked on My Computer and then I clicked on properties.
I clicked on Hardware and then I clicked on Device Manager.It fails to start.The a notification came on screen which says:-Windows cannot find 'mmc.exe.' Make sure
you typed the name correctly,and then try again.To search for a file,click the Start
button,and then click Search
I did what this notification said but all in vain.Everything remains the same.
I want to solve this problem to open the Device Manager.
What is the solution?
Thanks in advance.

When did this start happening? If fairly recent, use System Restore to go back to a date before it started.

Click on Start, All Programs, Accessories, System Tools, System Restore.

Device Manager is devmgmt.msc. Try that.