Making unpacked progs work on both WinXP/Win9x?

[Barry]

I've been reading a few unpack tutes for UPX and managed to follow them OK, BUT when I unpack in either WinXP or Win98 the progs will only work in the OS they were unpacked in?

The tute I followed was this one:

http://www.exetools.com/forum/showth...&threadid=3185

which uses a combination of Olly, LordPE and Imprec to unpack/fix. I wasn't unpacking that particular prog in the tute but used it to unpack another UPX packed prog, this prog works when I unpacked it but only in the OS it was unpacked in

How do I make sure my unpacked UPX prog (or any other prog for that matter) will work in both WinXP and Win9x?

I know I could use UPX to unpack, but I have a scrambled UPX prog that needs to be manually unpacked as none of the UPX descramblers work on it.

I'm guessing the API calls are different under Win9x/XP and they need fixing?

Any hints please?