Well- for starters, not all programs that run under Win 9.x are compatible with XP, period. There's not much you can do in that case except get an XP-compatible version of the program (if it exists).
In terms of something like VB, I'd imaging you would probably run into problems like version mismatches of the runtime libraries and/or other key components. What version of VB do have? Is it supposed to be XP-compatible?