Yes, this is exactly my problem. Context switching.
As a lot of people know, their productive regime can be destroyed by having to context switch - answering a phone you don't normally answer, changing the way you stack your papers, even switching between mouse and keyboard when playing a game like Counterstrike (I can subconsciously do everything with the buy keys on Counterstrike without problem, but the second I play a "automatic weapon spawn" game, I find myself trying to get into buy menus that don't exist, and taking my hand off the mouse to try to type two-handed when it's not necessary) - and that's exactly what Windows 7 and 8 do to me.
It's not even the fact that there are "changes", but the way those changes operate to make things different. This is also my exact problem with the ribbon toolbar - things change inconsistently until you've really, really used it a lot and then the advantage that those changes and memorising them give aren't worth the time it took you to learn them.
This is also my pet hate in browsers and other programs that pop up password dialogs - that's an entirely DIFFERENT beast that I need to know has changed, and the context needs to tell me that. I can't tell you the times I've typed a password (or more likely half a password) into a dialog that's popped up at random because of X and taken keyboard focus with no warning. How *dare* I look down at the keyboard for a second?! Fortunately, I have a long established habit of not pressing Return to login, so it's more a pain than a big security issue but how many old grannies type in their long, strong password and then look up only to find they've typed it not into their banking app but into a random website's "contact us" page?
Changes of context are THE key to an interactive desktop. I need to know when I'm going to break the system (i.e. for administrative modes, make you reboot into them, make all normal programs UNUSABLE while running them, and make everything go red so you *can't* use it for everyday use and certainly not without knowing - for general use, let things operate, don't let them steal keyboard focus EVER - for secure password entry, sound a note, clear the keyboard buffers, lock out all other programs from accessing the keyboard for the duration, ignore keystrokes with audible and visual warning if the user seems to have been typing from the previous screen, and then disappear once entered).
UAC was a very poor attempt at this that was worthless and seems to have corrupted people into thinking you don't need to do this. But you do. Desktop context is a big, major, design issue that if you get it right you never notice but if you get it wrong, everyone shouts. And, personally, that's exactly what puts me off a lot of software and the newer Windows OS. Hell, I'm just in the middle of filing a bug report on Opera because if you get an SSL warning while typing in another tab, it stops you being able to enter information and gets in your way. I consider it an horrendous bug for such an otherwise bug-free program.