Cannot log off NT Server 4.0 properly

I got an error message said “This Windows Application cannot respond to End Task request” when I tried to log off my NT 4.0 SP6a server running Exchange Server 5.5 SP4. The title of application is just a “W”.

It happens all the times except right after I reboot the server. Also, when it happens, I cannot run a 16 bit Setup program, which I must in order to update the virus list. All the files required by WOWEXEC are in the SYSTEM32 directory. It becomes very difficult to reboot the server everytime I need to update the virus list.