Ummm....Windows has been on mobile phones for quite some time.
Trying to get full PC type Windows on a phone is next to impossible.
As previously mentioned, the speed lag would be so bad that it would take forever to do any simple task (the speed of the phone's processor couldn't come close to even a old slow P2 processor).
You have to remember, these are suppose to be phone's.....you know..for making phone call's? Everything else is an ad-on.
Smart phones have come a long, long way...but are not even close to a PC, and really are not made to be that way.
They are a short term use device till you can get to your PC to perform the actual work desired.