Windows XP
What do I need to do?
I need to 'erse' hard drive.
I have the disk.
What needs to be done so computer will boot from that disk?

When your computer first boots you need to go to the BIOS screen. Most computers will tell you which button to press to get into the "Setup" when you first start the computer. Some Computers use the "Delete" key which is most common, others however will use the "F2" the "Esc" and sometimes the "F10" key. You'll have to figure out which key your computer uses on your own.

Once you are in the BIOS Screen you need to locate the boot order. Sometimes the boot order has it's own tab across the top of the screen sometimes it's under one of the configuration tabs. It really depends on what you have for a BIOS. You need to locate wherever your boot order configuration is.

Once you have located the boot order you need to change it so that the "CD-Rom" is higher on the boot list than the hard drive. A good Configuration would be.
First: Floppy Disk A
Second: CD-Rom
Third: Hard Disc (or Hard Drive again depending on BIOS)
Fourth: Not usually needed in everyday use.

Now you need to make sure you save your changes and exit. Be careful though because you are messing with settings that could potentially screw up your computer. Be sure not to change anything unless you know what it is you're changing.

Hope this works, Good luck.

Sometimes, you have the option to press a key for a 'boot menu' also. This would be kind of the quick and dirty way to go about it, but it gets the job done quickly. I see it mostly on Dell computers, where usually, when you boot up, it says "Press F12 for boot menu". I've also seen it as ESC, F9, and F10. This option isn't available in all computers.

Blaze's post is probably the better way to go. That way, it should be ready to just pop the CD in, if you ever need to do it again.