Not a thing wrong with that, i have had protools 5-6.4x LE digi 001 running on XP circa 2000..... still have the same digi rig, started out with intel 1.2 Ghz with about gig of ram....up until about 3 years ago, since then i upgraded the processor to intel 2.1 Ghz and new motherboard with 2 gigs of ram.....
I have absolutely no problem mixing, adding inserts, or dumping with protools i usually max out my available audio tracks on a lot of my joints....works fine, my thing is a couple of producers i kick things around with and work with use macs and have been just as long and I have seen for myself, yes a mac is not flawless.....the only problem i had was my drive crashed once, but other than that i dont have any problems.....
prior to that i had echo mia.....and delta 1010....on Windows and never had a problem...its all on using sound gear....XP is very stable if you are not using it to surf and getting spyware or other malware on it......if you do that will effect the performance of the DAW though....(i do everything on mine though and still solid)
As a side note I know one of the part owners of guitar center, and about a year or so ago i went to his house.... he had two g5's but he was using one of them strictly for plugins, he was using digital performer or some other daw i cant remember.....that just goes to show he wasn't able to do everything on 1 machine also but he had a ton of midi stuff connected....
1 last thing, when i create a session there is an option to make the project mac compatible anyhow....so for me there is not a big motivation beyond the masses hyping it and the industry glued to go to mac but i think 6.4 was the last version of protools le supported on Windows for the 001 maybe not the others though like 002 & 003, i think that was a mistake IMO because PC's are far from what they were 5 or 6 years ago in power and capabilities, like i said XP is underrated as far as stability, when you compare look at the ability and range of hardware customization available to do like you said, make a diesel machine....and for far cheaper than upgrading mac gear.