You can go here to download the ORIGINAL version that belongs on your pc and is written on your COA.
Official Windows 7 SP1 ISO from Digital River « My Digital Life
Download the iso , 64 or 32 bit and burn to dvd with Imgburn at 4x speed. ( most likely 64 Bit )
The Official ImgBurn Website
Clean Install Windows 7
How much memory does the pc have and what processor ?
Start>Right Click Computer> Properties