1. Besides un-plugging the power cable or sata cable from the hds
2. you can use VHD. Boot from VHD (Windows 7 Ultimate/ENT only)
Boot from VHD is a new technique for installing and maintaining operating system environments. Unlike virtual machines, the operating system that is running from a “boot from VHD” environment is using the actual hardware instead of emulated hardware.
Instead of dual booting (installing different OS on different partitions) or running multiple OS's at the same time with virtual machines you use a VHD. You choose which OS to start on boot. You can use linux too with VHD (not sure, I have not tried it myself.) Can I boot Linux from a VHD?
3. Use a virtual machine. Run one OS inside another with Virtual Box, Virtual PC or VMWare. You create a VHD/VM of the hd and run it from within the OS. Two separate machines running at the same time. Boot from VHD runs only one OS at a time, EDIT: Can not boot VHD within a VHD. But you will need to upgrade OS to ENT/Ultimate to use it.
It is very clean, as the entire OS is in a VHD file. Having the OS
fully encapsulated in a VHD means no changes are needed to partitions,
and there is no confusion as to what files are for what OS.