Computer rarely boots and I need to reformat my hard drive?

My computer rarely boots and when it does it crashes with the blue screen after a few minutes. I need to find out how to reformat my hard drive before my computer crashes. I am on Vista which I now realize is the worst operating system and I need to change it to XP fast. I know I could change it to boot from CD but I don't know how to do that. Puting in the CD and restarting doesn't work. I put together this computer so it's different. At startup screen it quickly says press tab for BIOS POST. but when i do that it either crashes, goes to the options for safe mode or run windows normally, or it freezes. What should I do?