Dual boot HOWTOs

Ok. Here are some of the most common dual-boot scenarios these days:

• You want to dual-boot XP and Vista (with XP installed first)
• You want to dual-boot XP and Vista (with Vista installed first)
• You want to dual-boot XP and Linux (with Linux installed first)
• You want to dual-boot Vista and Linux (with Linux installed first)
• You want to dual-boot Vista and Linux (with Vista installed first)
  

Now all of these things can be troublesome if you don't know what you're doing, a bit about MBR, bootloaders etc. However, there is an answer (probably quite a few)-

You can probably find all the info you need here:
The definitive dual-booting guide

also, a nice related article:
Preparing for dual-booting by doing an image-based backup