To elaborate, it looks like you're using IIS (based on the "Inetpub" directory); with IIS you'll have a separate "worker" account that the CGI/ISAPI modules will be executed under. Usually, this account is called IUSR_Computername, where ComputerName is the name of the computer running the server (unless you renamed it after IIS was installed, which I've done before and forgot about :p).
What you'll want to do is to make sure that this user account (or possible a different account, depending upon whether IIS has been configured to use different credentials for its worker processes) has at least "Write" permissions in the ACL of the folder you're trying to write to. As a matter of personal opinion, I usually just give the IUSR account full permissions on the Inetpub directory (and subdirectories).
