That is something thats a bit far away and is very limited. Im assuming you think everyone will just have a terminal and your computer is at the "virtual pc location".
Also, you will need an OS to see that virtual PC which of course will be Microsoft. I can see it now "Microsoft Windows VT (Virutal)".
I would NEVER in 100000 years trust another company to hold all my data and my "virtual pc". You would just need 1 insider to give some info to a hacker and 1,000,000's of peoples PC's will be corrupted, copied and exploited.
I have 4 servers with a T3 and use one of them on virtual desktop. I have tried (as an experiment) to play say a good online game like everquest over that computer while using Virt Desktop. Let me tell you, the word slow isnt even the right desc for it. And this is a T3 we are talking about..
I would say it will be a good 8-10 years before any kind of stable virtual desktop to go public (where it can actually cut into windows OS profits). For one, every tracert hop would have to have an outstanding amount of bandwith, this also includes your current ISP which isnt about to let anyone have higher upload speeds anytime soon. You need high upload speeds as well as download speeds to use any effective virtual desktop.
To avoid what i said above they would have to make some kind of direct connection to your home. Maybe if everyone was hooked directly into fiber optics (which i know is comming my area in the next 5 years) it maybe plauseable.. However, by the time it hits the main stream.. MS will own the software to make it and run it. You really think people that are goin to offer this to the general public are going to use anyone other then Microsoft? The general public only knows of them.. So it will sell quicker.
Microsoft will always be the main company or parent company to the largest share of the computer os market - regular or virtual.