Since you failed to include your PC model number we can't give you specific instructions. 

First, backup any user data, including wordprocessor files, pictures, music, etc.  When you reformat the hard drive everything will be gone.  You can't backup programs as they must be installed using the programs installation program.  Back up the data to data CD's or DVD's (depending on what burner drives you have) or if it's only a small amount of data you can back it up to a USB "key" type device.

In general the installation sequence on a "clean" (reformatted hard drive) install is (1) Install Windows (2) Install chipset drivers (3) Install device drivers such as the sound and Video (4) reinstall programs and backed up user data (5) reinstall preriperals (e.g. printer) and VERY IMPORTANT, disconnect any peripherals BEFORE doing the reinstall and ONLY reconnect them when the install program for the device tells you to.

If you don't have the Operating system CD, you can order it from Dell, HERE

Any drivers can be downloaded from the Dell downloads (click on Support at the top of the page).