You don't have to replace any Apps. XP was a monster when it came to this.
Vista is your best friend in this regard.
You are VERY misinformed! There are NUMEROUS reports of applications and
utilities not working on Vista, or requiring an "upgrade" to run on Vista.
Moving from Win9x to XP was a MAJOR step for most home users. it was a move
from a 32-bit overlay on 16-bit DOS, to a true 32-bit OS. Those of us who
evolved to XP from NT and Win2K didn't have anywhere near the application
compatibility problem, because the apps we chose already, for the most part,
worked on XP. However, the evolution of all the hotfixes and service packs for
XP tended to "break" a lot of older apps, so we upgraded as the need arose. I'm
STILL trying to fix macros and other stuff in Word and Excel from when I was
forced to upgrade from Office 2000 to Office 2003 just 2 years ago, after
several years running fine on XP.
In contrast, Vista is another 32-bit OS, and there should be virtually NO
incompatibilities with apps that run on the latest evolution of XP. Since MS
had already introduced the alleged universal hardware driver model with XP,
there should have been few, if any, driver conflicts, either. I will admit the
exceptions for games and other ill-behaved apps that try to access hardware
directly, because Vista does offer more protection in that regard.