10 hrs is good.
But a suitable ARM system will give same Browser/"Office Docs" performance and better HD performance than Atom with 1/2 weight of battery.
If you use the 2 x2 pixel RG BW staggered cell LCD instead of RGB inline triad, you save more power, have cheaper display and brighter panel. You might then get down to 1/3rd or 1/4 as with 10" screen the backlight becomes significant power drain.
Win7 (aka NT 6.2?) won't work on an ARM. Once windows NT did work on MIPS, Power PC, Alpha, 64bit Alpha, x86 and Pentium Pro. That was before Win2K. (NT5.0)
WinMo/CE works on ARM. But really it's a completely different OS, more different than Win3.x/Win9x/WinME family is from NT/XP(NT5.1)/2K/Vista/Win7 family.
Our penguin loving friends are likely. But not the Android flavour as it's GUI is designed for handsets and it's App based designed for Google's version of Java. A Netbook wants a more featured GUI and more native applications (plenty of both available for ARM + Netbook size screen).
Symbian is fine if they ditch S60 etc and do a new decent GUI in QT as rumoured, but you'd not bother with it on a Netbook. Nokia "PDA" Tablet Skunkworks stuff is already Linux based (N800 and related pocket tablets)