The site fullversionforever.net you mentioned is for Microsoft software. Downloading cracked or "highly compressed" OS images from such sites is extremely risky.
If the file isn't Windows 7, what are you downloading? usually, these "highly compressed" files fall into three categories: The site fullversionforever
A legitimate full Windows 7 installation is several gigabytes (typically 2.5–4 GB). A 100 MB file cannot contain a working operating system. What it could contain: what are you downloading? usually
Downloading Windows 7 (or any software) without a license or from unauthorized sources may violate copyright laws. The site fullversionforever