This article could sched a light on your problem:
Especially the part:
"Because using the FSO on the client side may provide potentially unwelcome access to a client's local file system, you should use it only in scripts executed on the server side. Internet Explorer default security settings do not allow client-side use of the FileSystemObject object. Overriding those defaults could subject a local computer to unwelcome access to the file system, which could result in total destruction of the file system's integrity, causing loss of data, or worse."