Do you mean to ERASE everything on the hard drive and then re-install the OS and all the drivers from scratch?
Before you do any of the follwing, be ABSOLUTELY SURE that you have access to the drivers you'll need for your computer. If you don't have access to the original CD/DVD, you can download them from here:
You will also need the SERIAL NUMBER or "PRODUCT ACTIVATION KEY" for your copy of Windows. If you have misplaced this, you can try the following program, which will read the product activation key from your HD:
There are several programs out there to erase the contents of your HD and re-format it.
A) One option is to use your OS CD/DVD:
1) Place your OS CD/DVD in the CD-ROM and boot from it (you may need to change something in your BIOS so that the computer does not boot from the HD.
2) After the CD boots, choose "install OS" (do not choose "repair installation").
3) You will be asked if you want to DELETE the partition in your HD that has your OS. Choose "YES". This will delete ALL information previously stored in your HD.
4) Proceed with the OS installation, following the directions on the screen.
5) After the OS is installed and your copy of Windows is validated, install all the drivers
YOU ARE DONE
Please, report back if you need further assistance