My Preference - If you have two HDDs
Only have one HDD connected. - Load operating system on that drive4. Once installation completed, disconnect drive and connect 2nd drive. Install 2nd operating system. Once that operating system is install. Reconnect first HDD. In bios select which drive you will use the most as your Primary boot drive. When you want to switch operating systems, Select restart, during the post hit the key that ask you which drive to boot to and select the other HDD. Can also do this when first powering on.
NO SOFTWARE boot manager to get hosed up and mess up both drives.
Note Which ever drive you select to boot to will be drive C, excluding other partitions the other drive will be Drive D. Drive leters switch when switchin HDDs.
Had my E6400 set up to triple boot this way - XP, Vista, and Win 7 RC (Since removed Win 7 RC.