Okay. on the MS product sticker it will say what type of Windows version it is ie: Win 7 Pro 32 bit, you can navigate to this page
and download the correct version, then create a DVD with it.
Then you need to look at the manufacturers site and download the specific drivers for that computer. The model number can be found underneath the battery (pretty sure this is a laptop right?)
You will NEED:
- chipset drivers
- lan drivers
- wireless drivers
- graphics drivers
Other drivers and downloads will be listed, but are often not necessary.
Install Windows (delete all existing partitions and let Windows create its own, during the installation process), register using the product key on the sticker, install the drivers starting with the chipset drivers, download and install the 130+ updates since service pack 1 (win update service)...