If you just want to format your computer, follow the following steps, they'll be the easiest I can think of with out a stick of TNT. I'll even give you two options, depending on the hardware you have available. Both require a second computer, but one only requires the ability to burn a CD, while the other take at least a little hardware knowledge.
A) Using windows to wipe it. This can be done with pretty much anyone's computer that has windows, though they gotta trust you at least a little, or you them.
1) Find a computer you can add another hard drive to.
2) Remove hard drive you want to format from first computer
a) Power down and unplug computer
b) Remove side panel from tower
c) Locate hard disk (normally near the front panel in a shelf about half way up the case)
d) remove mounting screws and ribbon/power cables (just ribbon if it's a SATA connection)
e) carefully remove drive from enclosure
3) Insert drive into secondary computer, following removal instructions in reverse order and procedure.
4) Boot secondary computer
5) Open the start menu and right-click on (My) Computer, then click Manage
6) Locate the Manage Disks option and click it. In the top window on the right side of the window, locate your drive. It will probably be the letter furthest from A:\, and compare the drive size to what you expect (you usually loose about 5-15% from what the box says, my 500 gig only shows about 465 gigs). If there are multiple drives with the same size, you can open (My) Computer and look through the files until you find the right one, just to be certain.
7) Right click on your drive and locate the Format option, click it.
8) In the Format Drive box that comes up, tell it to do a FULL format, not a quick one. Then click format.
9) Wait. It takes about 10-30 minutes, depending on the size of your disk drive.
10) Follow steps 2) and 3) to replace your hard drive into the original computer.
11) Boot computer with OS disk in the CD drive, and run your OS installer of desired flavor.
12) Immediately install antivirus, popup blocker, and spyware removal tool of your choices. I like AVG, SpyBot, and Stopzilla. They are all free and receive regular updates.
Your second option is a little more computer illiterate friendly, in that you do not have to crack anything open. Unfortunately, this method only works if your computer is capable of booting from USB. If your computer is less than 2 years old, hi probability that you can do it. If it's older, it gets iffy. You also need one blank CD.
B) Reformatting by installing Linux as a placeholder OS.
1) Go to a computer capable of burning at least CDs.
2) Download the Ubuntu Linux distribution at this link:
http://www.ubuntu.com/desktop/get-ubuntu/download Version does not matter. I suggest the 32-bit, simply because it will install on any system, where as a 64-bit won't. And you won't be keeping it.
2) After downloading the file (might take upto 20 minutes on a cable connection, more if you're using *shivers* dial-up), burn it to disk using a free program like Nero, or ImgBurn (my preference, personally) with the Burn Image command (NOT burn file, that will just copy it. A .iso file is an file that has all the files of a CD/DVD in it, with instructions on where to place them on the disk so that they will read right.)
3) Reboot the computer with the Linux CD still in the drive and tell the computer to Boot from CD.
4) use the "Try Linux without making changes to your computer" option. Should be the first one. This will boot Linux from the disk, rather than trying to install it.
5) Make sure your USB drive is attached to the computer.
6) Once Linux boots up, go to the System menu.
7) Go to the Administration option.
8) Click the "Create a USB startup disk"
9) As long as it tells you that you've got the room on your thumbdrive, click "Make Startup Disk" button in the bottom right corner.
10) Once that finishes, take your thumbdrive and go to your computer, attach it and boot.
11) As your computer boots up, press the button it requires for changing the Boot Device.
12) Tell your computer to boot from your USB drive.
13) Have it install Ubuntu, telling it to use the Whole Disk. That will format your drive, giving you a fully functional OS.
14) Simply reinstall original OS.
Hope this helps a lot.