If you want to have dual boot on your pc, for instance if you want to have Windows 2003 R2 installed side by side to run demos and requiring server products that do not run well in a VPC, then there is a few thing you need to be aware of.
First of all, Windows Vista has introduces a new bootloader called Boot Configuration Data (BCD). You can read more about BCD here: http://msdn2.microsoft.com/en-us/library/aa362639.aspx. This also means that the old boot.ini is not used anymore.
If you started by installing the "legacy" OS you should have no problems but if you installed Vista first and then your “legacy” OS after worth, then the “legacy” OS will overrule the BCD and you will not be able to boot into Vista. Adding Vista to boot.ini won’t help either as Vista cannot load from boot.ini.
So what you need do if you have brought yourselves into this mess is to boot up in the “legacy” OS and insert the Vista install DVD. Open a command prompt and browse to the boot folder of the install DVD. Here you will call the command “bootsect /NT60 ALL”. This will recreate the BCD and the next time you boot Vista will load!
But what about my “legacy” OS you might think! Well, here you need to modify the BDC and to do that I will recommend a tool called VistaBootPRO 3.1 http://www.vistabootpro.org/. This enables you to edit the BCD through a nice GUI and all you need to do is add an entry and be sure to point the boot drive to where Vista boot from and not the “legacy” OS.
And just for the record: I have intentionally quoted legacy as I don’t find neither XP nor 2003 R2 to be legacy OS…