Unknown users in Windows 7 may appear due to software installations or as remnants from previous setups, often linked to GUIDs in the registry. Users can reclaim ownership and delete these accounts if they are unrecognized, and it's generally safe to do so if the system operates normally. If unknown accounts reappear, checking the GUIDs can reveal their origins, potentially indicating software references. Additionally, if a computer was previously part of a domain and is now disconnected, it may show unknown accounts due to inaccessible active directory information. Monitoring the situation over time is advisable to ensure no further issues arise.