If you want to remove the OEM (Original Equipment Manufacturer - i.e. Dell or HP) logo then all you need to do is delete (or rename) the OOBE.XML file from the C:\Windows\System32\OOBE\Info folder.
If you want to change the OEM logo then you need to edit the OOBE.XML file and do a search for the term 'oemLogo' and then change the path/file to where you've saved your logo file. Make sure it's a .png file though else it wont work.
