Windows 7: Reinstall windows 7 from product key only

My copy of windows 7 has become messed up. Manually deleted a perental control software (son outgrew it) and messed up windows. I have run the system backup onto a CD and have backed up all files and data on an external HD. I've been told I need to reformat the C: partition and then do a restore. How do I reinstall windows 7 using the product key listed on the computer. No disks came with the computer. Win 7 was preinstalled by manufacturer.

What is the make & model?

This may be helpful:

Clean Install : Factory COA Activation Key

Hp g60-519wm.

Pressing the f11 key during startup on a computer with an HP factory image will start the system recovery process even if the prompt is not displayed.

How to make HP Recovery DVD disks:
Recover Windows Vista Operating System Using HP Recovery - HP Customer Care (United States - English)

How to make HP Recovery USB disk:
Creating a Recovery Disk on a USB Flash Disk HP Pavilion dv6700z CTO Entertainment Notebook PC - HP Customer Care (United States - English)

or
You can Order HP Recovery Disks from here:
Compaq Mini CQ10-500 PC series*-* HP Notebook PCs - Order Recovery Discs for Windows 7, Vista, or XP - c00810334 - HP Business Support Center

This is a Custom-built rig which came with Windows 7 preinstalled so there's a COA sticker with Product Key?

If you have no Factory Recovery disks or don't want to order them, then you'll need to borrow or find a Windows 7 installer to clean reinstall with Product Key on COA sticker following these steps to get a perfect OEM reinstall: Reinstalling Windows 7