Two O/S in one PC

[alrot]

I got Xp and win7 , I was wondering ,there are many positives things in both  .

Many  programs & hardware doesn't work in 7 but I find that FSX runs a little bit faster in 7 , I have plenty space  (1.5 Tb) in three hdds SATA

I'm worried to do such instalation ,win7 invades everything and may put files in my xp instalation in the other HDD , Is there a way to prevent that?