If you have installed two or more copies or versions of an operating system on your computer, and if you want to re-boot into another OS from your current one, you normally follow the following procedure.

Press restart, wait for Windows to shut down, wait for your BIOS to post, then finally select the operating system you want to boot into within the bootloader time-limit.
iReboot from NeoSmart, is a simple yet effective reboot helper tool. iReboot sits in your taskbar at startup, taking up only 400KB of memory, and lets you choose which operating system you want to reboot into. You just select the OS entry from it and let it do the rest!
Quite a handy app if you switch your Windows frequently from say, Windows 7 to Windows Vista and vice versa.
If you have liked this post, you might want to check out some more, on topics like Freeware.