first thing, if you plan on doing that, back up all your files that you do not wish to lose (copy them somewhere safe). This is an important step because after you install win7 all of your files will be gone.
do you have a CD that came with the computer when you bought it?
if yes, put it into the cd drive and reboot the computer, during start up it will ask you if you wish to boot from the CD, select yes, at that point win7 installation should start, so just follow on-screen instructions
if you don't have a CD, there might be a recovery partition on your hard drive. Refer to your user's manual to find out how to use this recovery partition to restore your system to its original settings.