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Following is the key which is responsible for it to work or not work
HKEY_CURRENT_USER\Software\Microsoft\Win
OR
HKEY_CURRENT_USER\Software\Microsoft\Win
Look for 'NoRun' its a DWORD with default value of 00000000 (0).
In your case it could have been changed.
Change it to original default value...!
Suggestion: U can do it also by gpedit.msc!
Steps:
1. Open Run n type gpedit.msc n hit enter...!
2. Go to the left pannel : "User Configuration > Administrative Templates > Start Menu and Taskbar > "
3. Towards ur right u'll find several options ....serach for.."Remove Run menu from Startmenu"...!
4. Disable it and if it is already set as not configured , enable it and then disable it .. !
5. U r now done....!
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Enjoy...