Re-installing Windows is often necessary, but few people have the DVD, and many computers don’t even have a DVD drive anymore. Microsoft provides free, legal downloads of Windows 7 for installation from a USB stick — or burning to a DVD, if you want.
These downloads require a valid, legal Windows 7 key (serial number) and the key must match the edition you download, i.e. if your key is for Home Premium 64-bit, it will only work with that edition, so that’s what you have to download.
If you don’t know whether you want SP1 or not, you probably do.
Windows 7 Home Premium 32-bit
Windows 7 Home Premium 64-bit
Windows 7 Professional 32-bit
Windows 7 Professional 64-bit
Windows 7 Ultimate 32-bit
Windows 7 Ultimate 64-bit
Without SP1 (Release To Manufacturer version):
Windows 7 Home Premium 32-bit RTM
Windows 7 Home Premium 64-bit RTM
Windows 7 Professional 32-bit RTM
Windows 7 Professional 64-bit RTM
Windows 7 Ultimate 32-bit RTM
Windows 7 Ultimate 64-bit RTM
If you prefer to download Windows 7 SP1 manually, you can do so here.
Once you’ve downloaded the file you need, see here for instructions on creating the installation media (USB or DVD).