DOS prgms not running.

Hi,
I am runnig a novell Server. In the login script of novell, I am running a DOS program. When users are logging into the server from Windows 98 machine(client side), it is opening a DOS winodow titled ‘NWLSCON’ and runs. But some of the machinesit is not happening. When this DOS window comes it gives an error of illegal operation and ask for reboot the machine. This is happening only when the user is logging in the first time, ie if he loggs off and login again the problem does’t occur. Kindly give me a solution for this