Ok, I cracked and emailed jeff@windows.com from that PC advert. Because I’m a massive geek.
I got a reply!
Hi –
Thanks for your email. I apologize if I can’t send you a more personalized response than this auto-reply – I’m super busy these days trying to optimize my code & squeeze out that last drop of performance. Or sleeping off a caffeine crash 
In case you were wondering (you did email me, right?), I’m a software engineer working on Windows Networking technologies. (Networking, like, wireless internet and all that}.
I’ve been getting a lot of questions so I just wanted to take the opportunity to answer a few. Several people have asked why I get an email address in the TV spot when most of the other folks don’t.
The answer is that I’m a Microsoft employee – the others are real people too, but they don’t work for Microsoft.
Another fun one that keeps showing up in my inbox… can I apply for a job at Microsoft?
Yes! The Windows team is hiring ;-)… check out http://microsoft.com/jobs/ or http://microsoft.com/college/ (for students).
And thirdly, people are asking a really great question: am I real?
The answer: Yes, at least last time I checked.
OK. I really need to get back to work now, so bye & thanks again for your email.
–Jeff
So is Jeff real? I think so, but I bet after that advert he had to change his email address