Are you sure that RMXP was never installed there? That message can happen in two cases: if the setup fails to write into the registry, or if the registry already has an older date from launching the trial earlier.
It might also be incompatibility - RMXP was written for Windows XP (which basically works 32 Bit) while Windows 10 is not only a dozen years newer, it is 64 Bit. Have you set compatibility to WinXP?
There already have been several quirks with RMXP on Windows 7, specifically because the structures of Windows changed since WinXP, and it's entirely possible that W10 added more problems for that.