most of the time people will do backup to their files before moving to another platform. it is for safety.
if it is me, I would rather stick to my windows xp.
pentium 4 do support vista, but it will be great if you have more ram on your pc or else it will lag..
stick to windows XP is better for Pentium 4. Not just Pentium 4, the device drivers comes with that machine may not all vista ready.