Removing Windows 8 could screw up your bootloader and prevent your original OS from starting properly. Delete the Windows 8 volume and extend your remaining partition into the freshly unallocated space. If you want to get rid of Windows 8, load your primary OS and launch partition software (again, Disk Management for Vista or 7 users). If you're having trouble launching the USB drive, you probably just have to put it ahead of your system drive in the BIOS (look for boot options). Just boot off your Windows 8 USB drive, choose a custom installation and select your newly created partition. It should be smooth sailing from here as Microsoft's installation process guides you through everything. Windows XP users will have make partition adjustments with a third-party tool such as Partition Logic, but you'll do the same thing: shrink one volume to create another.
Right click the drive that you want to house Windows 8 and choose "Shrink Volume" (Windows 8 64-bit requires at least 20GB, so shrink your current partition accordingly).Once the application loads, you should see your operating system's drive. Vista and 7 users can create a new partition with Windows' Disk Management application (Start > search for Disk Management). The latter, however, requires a little more effort. The former is relatively self-explanatory, just attach the drive and choose it during the installation process.
If you want to dual boot Windows 8 with your existing operating system, you'll have to install a second storage device or create a new partition. However, Windows 8 won't save programs from Vista and it won't save programs or Windows settings from XP.
Windows 7 users should have a painless upgrade as programs, Windows settings as well as user accounts and files are imported. If you plan to upgrade or overwrite your installation of Windows XP, Vista or 7, open the root directory of your USB drive in Windows Explorer and launch Setup.exe to begin. Needless to say, back up anything important before proceeding.
Install the Windows 8 Release Preview (key: TK8TP-9JN6P-7X7WW-RFFTV-B7QPF)