A bunch of people have already chimed in, but here's another, in case you're still looking. This method works for dual-booting any (Windows) operating system, as long as you have the ISO. I've used this since the Vista days, first to tryout 7, then restore it many times, then to try out 8, and now for 10. I don't carry CDs so I
boot and install from a USB drive.
1. Get the ISO from
Windows Technical Preview - Microsoft Windows. Make sure you get the one for your system (x86 or x86-64).
2. Format your USB drive. Simply right-click on the drive in My Computer/This PC and select format. File system should be FAT32. Use Quick Format.
3. Download this tool from Microsoft:
Microsoft Store. (I know the link says Microsoft Store, but that's WPCentral trying to be smart and guess what the page name is). Install it and run it. Point it to wherever you kept your ISO and then point it to your USB drive. It would extract and copy the ISO to the drive.
4. Partition your disk. A nice fellow up ahead already has directions for this, but for some reason Windows doesn't see all the free space on my drive. I use
Aomei Partition Manager. Install and run. Select your hard disk, and using the slider pick how much space you want your new partition to be. Microsoft recommends 16 GB for 32-bit Windows and 20 GB for 64-bit. I usually add an extra 10 GB for the pagefile and hiberfil and any programs you might want to install. For 64-bit Windows, 30 GB is a good amount. When you're done hit Apply up in the top menu. Unfortunately Aomei requires a restart to partition your disk, and it takes some time, but for me it was the only option because Disk Management simply wasn't working.
5. After the reboot launch Aomei again. Right-click your new partition and select Create Partition. Pick a drive name, set File System to NTFS and hit OK then Apply.
6. Now restart your PC. Select the USB drive as the boot drive in your BIOS (this varies from system to system) and watch Windows 10 install.
Remember to pick Custom Installation, and select the new partition you just made, or the installer will upgrade your Windows 7 or 8 installation.
7. Go through all the setup screens and enjoy the beauty of Windows 10 (I hate the name, BTW).
8.
Bonus: Go to File Explorer, select View->Options. In the Navigation Pane area tick "Show Libraries". Now customize all the Libraries to include folders from your original Windows partition, be it music, documents, videos, or even custom libraries. Now you have native-ish access to all the files from your old PC. Windows even indexes those folders so that searching for files is a breeze. Under Manage in the Libraries ribbon in File Explorer you can set these folders to be your default save location. Now you don't have to worry about making/using files on this partition and looking for them if you ever want to go back to your old Windows 8/7 partition.
WARNING: Making a partition does not remove all chances of system failure. I had no issues with the Windows 7 Beta and Release Candidate but the Windows 8 Consumer and Release Previews caused SMART failures on my hard drive (even with the partition) and that of a friend. Of course it's entirely possible that they failed all by themselves, but separate hard drives in separate systems running Windows 8 Preview and dying doesn't seem like a coincidence. The best thing to do is install on a PC that you won't mind getting destroyed or a virtual machine. This is pre-release software. Proceed at your own risk.
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