Kiki, Typically if a file has a _ at the end, it's a compressed file. The only files that you really look at with a text editor are inf's and sif's. As far as replacing system protected files and other files, you're best bet would be to familiarize yourself with this. If you are only slipstreaming the hotfixes listed on Tomcat76's hotfix webpage, you do not need to alter any files (like sfc_os.dl_). However, if you want to add in the tweaks such as xpize or certain AAO's, then you are more than likely going to need a modified sfc_os.dl_. FYI, xpize modifed a boatload of dlls.