Error while executing the 'exec'-command

Hi all, I have a problem with the 'exec'-command on a NT-machine. I use a MS-DOS-program that writes a file into the filesystem. The syntax is like: "prog.exe c:\test.txt". With PHP3 I write this line: $test2 = exec("prog.exe c:\test.txt", $output, $return_value); If the output-file locates in c:\... the file was created sucessfully. But if I will have an other local drive the DOS-program means that he can't write to the file. On a console the program can write the file. I hope you can understand me and have a solution for the problem. Thanks Frank