Older Windows 7 loaders modify the Master Boot Record (MBR). If the activator is poorly coded (or malicious), it can corrupt the bootloader. This results in the dreaded "Bootmgr is missing" or "NTLDR is missing" error, effectively bricking your OS until you perform a clean install.
Tools like KMS Tools Portable or KMSOffline use Microsoft's own enterprise activation technology to validate the OS locally. Filecr Windows 7 Activator
Widely considered the most effective tool for Windows 7, it emulates a SLIC (System Licensed Internal Code) to bypass activation. Older Windows 7 loaders modify the Master Boot Record (MBR)