SMPlayer 16.6 and newer doesn't run on Windows XP on old machines with CPUs without SSE2 support (Athlon, Pentium 3).
Error is "Application cannot be initialized". Same error is for SMTube 16.6+. Last working versions is 16.4/16.3 build with qt4.
Swapping system to a CPU with SSE2 support does the trick.
Since SMTube is only way for youtube on old PC, maybe it's better to remove SSE2 requirement or produce a separate qt4 windows build?