You need to install QNX to run AmigaXL. There is no alternative.
However, there is no reason to assume that whatever was crashing AmigaXL was CPU related. While it is true Amithlon was tested for Athlon processors, this is not necessarily the case with AmigaXL. The latter may have problems with P4 processors, because H&P have failed to release the source code that would allow a bug to be fixed, but this might not be what crashed AmigaXL previously.