Rather than using NPE to cross check the system security, I suggest you to use Malwarebytes Antimalware free edition from www.malwarebytes.org. Regarding the system incompactablity, you can always keep a backup of older versions of NPE and wait till this issue is solved.
Agree, and that's what I have been doing for years. I use MBAM, SuperAntiSpyware, Sophos, F-Secure and more. Plus, I use MB's Anti-Exploit as well. Afaik, older versions of NPE won't run, as the program checks for updates every time. Won't run offline either.
Frankly, if, down the road, NAV/NPE won't work with my old (SSE only) computer (I don't think this is an XP issue), I can live with that. But, my point is that the Symantec/Norton Support guys should come 'out of their closets' and explain what's going on. If NAV/NIS only supports SSE2 CPUs from now on, I think Norton should say so and be clear about it. Simple as that.
XP is dead and has been issued EOL and EOS a lot ago, so just using an AV to protect unpatched os will be not much effective.
My XP computer can't be upgraded to W7. So, until it's replaced with newer hardware, that's the way the cookie crumbles. But, I don't think Norton should be the decisive factor here.