I had some little problems in the past when installing P5 (demo version I downloaded - not longer available I guess) over Sonar 8.5, so when I got my hands on the full P5 version I decided it was time to do a fresh new clean Windows install.
At first I installed Windows 7 64 bit, drivers etc. and did some performance tweaks. Then I installed all free FX and synth stuff (still love Independence free, NI Komplete 7 players, Proteus VX, Voxengo stuff etc.) And on top of that P5. I did not run the application, but installed the upgrades before doing anything. Then I launched P5 and did the usual configuration stuff.
After I was sure P5 (v2) worked like it should do, I installed X1 on top of that. This installation replaced some old versions of stuff used in P5, but it does not seem to have any bad effect on P5 itself. The full fresh version of Dimenson Pro works great in P5. And before someone asks - no I did not installed the older Sonar 8.5. I only use X1 now. The only reason I keep the 8.5 install, disk around is because it has the Lexicon Pantheon Verb on it. This one is missing on the X1 install DVD (guess it is for licence or stability reasons, although I do not have any stability problems after installing it from the 8.5 DVD).
The result is I have a perfect working P5 together with X1. Oh - and this time I did not even bother to install Ableton Intro. P5 gives me all the "scratch-pad" usability that I was using in Intro.