What you are describing is a dual-boot system and as Grumpy Trucker states Vista will start to load first before asking which o/s you want. This is not necessarily the best way to do it.

As you have two hard drives you may be better off getting a 'boot manager' which appears on your screen when you first switch on and asks you which drive you want to boot from. This has the advantage of both systems being completely independant of each other and NOT changing the MBR on the primary drive.

I have this set up and with my boot manager I press F8 when my PC is switched on and then get a list of drives (not operating systems) to choose from.

Acronis is one of the best...but not free I'm afraid....This is the only "free" Manager that looked anything like decent..
it's a very simple basic interface ..I've heard it does the job...and it may be what you are looking for..click here

Have a look at the details on the site and see what you think..I'm sure you already know that Boot Sector changes can leave you unable to access your O/S if you make a mistake..so approach it carefully and take your time.:)

I'm obviously doing something stupidly wrong trying to dual boot the XP Pro and 7 RC with EasyBCD because I failed dismally.
I have now removed the 7 RC drive and installed it in my second machine running Vista. Computer shows Vista on C: and 7 RC on D:. Downloaded and installed EasyBCD on C: but I still do not have a dual-boot facility.
Any advice out there?

I bit the bullet, formatted drive D: and re-installed 7 with C: connected and running Vista. Needless to say, I ensured I had an Acronis backup of Vista beforehand.

I now have a choice, via the EasyBCD boot manager, of Vista or 7 RC. Looking forward to my pre-booked copy of 7 later this year and feeling more confident about it.

No special software is needed. I have Vista & W7 on two separate drives. I can press F12 when the BIOS is loading and it shows each hard drive and I can then select which one to boot from. I prefer this as I don't want to get prompted each time I start up/reboot if I'm not changing OS's.