Generic RootKit.x is a detection for several specific Trojan variants.

Rootkits are programs (device drivers) that can potentially be used with any malware to hide, or stealth, files, processes, registry keys, and network connections. Additionally, they make it harder to detect or remove the malware. This is one of the generic detections for such class of malicious programs.

As new Trojans are frequently added to this detection, users are recommended to use the latest engine/DAT combination for optimal detection. Exact details (filenames, Registry keys, file size) will vary between variants.

Typically, a Generic RootKit.x variant might install itself into the %systemdir%\drivers\ or %systemdir%, and register itself as a service and set to automatically run at startup.

More information can be found at this McAfee page.