How to fix This PC's processor doesn't support a critical feature SSE4.2?

It simply meas the cpu is too old when you come across the "This PC's processor doesn't support a critical feature (SSE4.2)." error message during Windows 11 upgrade from Windows 10.

SSE4.2 (Streaming SIMD Extensions 4.2) is an instruction set extension for x86 CPUs, introduced by Intel in 2008 with the Nehalem architecture and later adopted by AMD. It includes advanced operations for faster text processing, data comparison, and cryptographic calculations, with the POPCNT (Population Count) instruction being one of its key features. These instructions optimize tasks like string parsing, checksum calculations, and AI workloads by allowing the CPU to process multiple data elements in parallel, significantly improving performance in supported applications.

Windows 11 requires SSE4.2 (including POPCNT) as part of its minimum system requirements because modern operating systems and applications rely on these instructions for security, efficiency, and performance. Features like malware detection (via Microsoft Defender), file compression/decompression, and even basic OS tasks benefit from SSE4.2 optimizations. Without it, Windows 11 would either run poorly or fail to support critical functions, which is why Microsoft enforces this requirement to ensure compatibility with modern software standards.

How to fix This PC's processor doesn't support a critical feature (SSE4.2)

If you have enough budget, then upgrading the CPU to a newer one ( (typically Intel 2nd Gen Core i-series or AMD Bulldozer/Piledriver and later)) is the most direct and safe solution to fix this issue. 

For time-saving and cheap alternative, you can use dedicated app to bypass Windows 11 processor checks. This will help you install Windows 11 on unsupported processor.