ashnish_rahjan,
This really doesn't seem to be an installation issue. If you are able to call ffmpeg-php's functions it seems as if you have installed the application correctly.
You will have to check your IIS directives to see if your script has write permissions to the folder you are attempting to write to, see the quota, as well as see the system's users permissions for this folder.
If all permissions check out ok, I would check with ffmpeg-php's group page, and see the community that uses it to see if your syntax is correct, and furthermore to ensure that there is no bug in the software.
Best of Luck...