First of all, look for your Windows 7 product key which is usually of 25 characters written in an alphanumeric string. It might be in the instruction manual or might be in a keyfinder program like the Magical Jelly Bean Keyfinder.
If you have a good internet connection available, you can download an ISO file for the version of the kind of Windows you own. It could be Home Premium, Professional SP1, Ultimate SP1 in 32 or 64-bit versions. The downloads are over 3GB so it is best if you have a good connection.
Now, download Windows 7 USB/DVD download tool as it will allow you to copy the Windows 7 ISO file to a USB flash drive or DVD.
Using your DVD or USB flash drive, you can boot your computer. Follow each command prompt that pops out as it will prepare your destination hard drive and install windows.
It is not recommended to delete the recovery partition or throwing away or discarding existing recovery discs. Many users are not able to activate their copy of Windows 7 despite entering the product key. You never know if a procedure works for you or not so it is advisable that you always have a backup plan with you at all times.
Furthermore, before starting the installation procedure, make sure that you store all of your important data to an external media such as a portable hard disk. Reinstalling windows will clean all the data you have in your hard disk so it is going to be very wise of you to make sure that you take every important thing out of the designated drive where the windows will be installed.
In order to download a bug free copy of the ISO file, go to the Website of Digital River as it is the licensed distributor of Microsoft software.