Why UPDATE.SYS size doesn't match with the original? Your UPDATE.SYS: 60,576 bytes. Original UPDATE.SYS: 60, 592 bytes. Do you know any good free patching program (supports silent patching)? <{POST_SNAPBACK}> There is a totally free binary diff/patcher originally written for FreeBSD, but has been ported to Windows. See http://www.daemonology.net/bsdiff/ You will see a link to the Windows port on that page.