I'm assuming you're in Win9x with real DOS, not XP or NT/2K.
Locate the DLL file, remember where it is...
Now reboot to Command Line (NOT 'Shut down to DOS', do a restart and tap F8 until the menu comes up and select 'Command line'). Navigate to the DLL's location (you're going to have to use the short 8.3 names, like Program Files is progra~1, if necessary. Go exploring...). When you find the DLL in DOS, first ATTRIB -H -S -R library.dll, then either DEL library.dll or REN library.dll library.lld (preferably the latter, no Recycle Bin in DOS).
Reboot, and the DLL in question is deactivated (if you renamed it to LLD) or deleted. If the system takes a dump with it missing, now you know where to go change the extension back in DOS...
