such devices have existed for a long time. the reason you don't know about it is because a full featured OS sucks as a tablet OS. if you want a windows 7 tablet check out asus Slate EP121 Windows 7 Tablet
Like others have mentioned, tablet PCs have been out for a long time. But they suck shit because full OSes are not optimally built for such devices, whereas Android and iOS are.
Give it some time and you'll see Photoshop comparable - if not Photoshop itself - available for tablets. The market is still young, and development is gaining momentum. If you can't wait, there are some very competent paint apps available on iTunes already. Not sure about Android market yet, though.
It's a Windows 7 Professional tablet with a few interesting features like an Atom Z670 processor and a 1366x768 10" display. It is well equipped with 2 GB RAM, a 60 GB SSD, a digitizer pen and HDMI out. It is IP52 rated and is supposedly drop proof.