Sometimes windows wont recognize both operating systems.When this happens, I use the recovery console that's on all XP CDs,as far as I know.I'm not sure if its on Vista.
   I have 3 operating systems on 2 hard drives.On the first hard drive I have XP PRO.I have the second hard drive partitioned with XP and VISTA ULTIMATE.Here's how I do it.
1. Set your BIOS to boot from CD drive first
2. Put your installation C.D. in drive and reboot computer.When prompt comes up to hit any key to boot from C.D., do it.After a bit of loading you will be given some choices.1 of them is to press R on the keyboard to open Recovery Console,do so.
3. Your then asked which Operating system you want to go to.I type choice 1, this is my c drive.Once this is done ,you will be in command prompt.At this stage you can type in help and hit enter, which will give you a list of prompts that you can use.
4.type in bootcfg and hit enter.This will give you a list of bootcfg options.
  This is what I do
I type bootcfg  /rebuild. This takes a few minutes sometimes.When done it will show all the available operating systems.It will then ask you if you want to add (y/n/all).
I type A and hit enter.Then it asks what you want to call your entry. I type C for C drive, etc. It will ask about next drive I type D for D drive.When your done entering your drives, it will go back to Cdrive. Type  exit and hit enter.this will reboot computer.
Now if everything went well, when rebooting you should see the option to boot from Vista or the other drive.Boot back to Xp for now.
When back in Xp , ( I have my XP set on Classic View) right click MY COMPUTER hit properties,advance.You should now see STARTUP and RECOVERY.Hit settings.You will see Default operating systems.I change " time to display " from 30 seconds to 5 seconds.This way you boot quicker.
Also if you hit the button next on Default operating systems would should see what operating systems you have.
Hope this helps.

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