A general rule of thumb when multi-booting Windows OSs is to install the oldest version first, newer versions will generally find the old ones and add them to the bootloader.

However, if you've already got them installed and you just can't boot, and aren't ready to learn the ins and outs of GRUB yet as you seem not to be an experienced Linux user:

1. Boot from your Vista disc. You should get to a screen that asks you to install. Look below the big install option for some other options. When you choose to repair, you'll get some more options, one of them will scan for Vista and fix its bootloader.

2. Google "Easy BCD." Download and install it. From there you'll be able to modify Vista's bootloader to load other OSs, like your XP installation. You can even point it to Linux partitions to boot them, too.

3. If you do install Linux, towards the end of the installation, you'll have an option to install GRUB somewhere other than the master boot record (MBR). Choose to install it to your /boot partition.

4. Boot back into Vista and add your Linux install to Vista's loader via EasyBCD.