Even if the file only exists in driver.cab, it should still have a dosnet.inf entry. You can easily confirm that by looking in dosnet.inf for a file which exists only in driver.cab. I used 3dfxvs.dll and what do you know, it's in dosnet.inf It should also be noted that files which exist in both driver.cab and in the i386 directory (for example, rfcomm.sys) still only have one dosnet.inf entry. If there is more than one, it may cause problems (though I'm not entirely convinced it would).