Data Lost to an OS Reinstall
An OS installer writing a fresh system onto a drive is, from a data-recovery standpoint, one of the more aggressive things that can happen to it — not because it deliberately wipes everything, but because installing and then using a new operating system generates a lot of write activity in a short window.
What's actually happening
An installer creates new partitions or reformats its target, then unpacks and configures thousands of system files, writes registry or configuration data, generates logs, and often creates a swap or page file — all of it write activity, and exactly the kind that overwrites recoverable free space the fastest. If the system was used normally afterward — browsing, installing applications — that compounds the problem further with every hour that passes.
How recoverable this really is
How this plays out varies enormously, and time is the deciding factor. Recoverability depends heavily on whether the old data lived on the same partition the installer reformatted or reused, versus a separate drive or partition the installer never touched; and on how much the system was used after the fact. If the operating system went onto a separate drive and the old data drive itself was left alone, recovery odds can still be quite good — this scenario is sometimes assumed to be hopeless when it isn't. If the old data's own partition was the install target, odds decline sharply the longer the system keeps running.
The general approach
The same metadata-scanning and signature-carving techniques used throughout this catalog apply here too, but success is essentially a race against however much writing has already happened since the moment of loss. The priority is stopping and imaging whatever's left as early as possible, rather than continuing to troubleshoot on the live system.
The moment you realize a reinstall happened over data that mattered, stop using that machine and drive entirely. Every additional boot, update check, or background service running in the meantime is more writes landing on the exact space that needed to survive untouched.
Practical, step-by-step guides for specific recovery software will be added here as separate linked articles.
This page describes the general situation, not a guaranteed fix for your specific case. If you'd rather have someone experienced take a look before you try anything, see what a hands-on evaluation looks like.