Problem with opening drive/folder

I m facing a strange problem from last few days.
Whenever I clicked on any drive(i.e. C,E etc.), it is opening in new window.Every drive's default open with option is changed to "AUTO" from "OPEN". Though folders are opening in the same window.

Also autorun.inf file which I have placed in each drive in not displayed (not even show hidden-system files mode) and it is not allowing me to replace that file with another copy of autorun.inf....

Is it any kind of trojan or wht???