The easiest way for us to get into the registry of an OS we can't boot into is to do it from another Windows operating system. All we need to do is have the partition of the target OS visible to Windows and assigned a drive letter, then we can use Window's own regedit utility to browse to and open the registry file we want to edit. Being able to see our desired partition should be easy on a multiboot Window's machine, but we may have to assign it a drive letter in Disk Management, or maybe even unhide it if it has been set as hidden by our bootmanager. If no multiboot is available then we can remove the hard drive and connect it to another Windows machine, either by USB or a SATA connection. Another option is running Windows from bootdisk or other removable device, or even just using the Windows Recovery Environment (WinRE) from any New-Gen Windows install DVD/USB - see TechBench box below for these last options.
Open a key.
How ever we get to a place where we can run the Windows regedit utility and see and browse the partition of our troubled OS, the details of opening the MountedDevices key for editing are as detailed below. If you need some help with using regedit then you can find the basics in this outside guide, but as a novice your first foray into registry editing should clearly not be on an important machine or operating system unless you have the ability to recover from disasters by having full backups and the means to restore them.
In our example we are booted into a Windows-7 OS and we are going to open for editing the registry of a WinXP install that we can see as the F: drive. It could however be the other way around or be with Vista/8.
Unload a Registry Hive File.
When you have finished editing your new key you should unload it from the Windows registry. Be careful to follow the correct procedure for doing this as it is surprisingly easy to inadvertently just right click and delete it, which is not what you want to do.