d) It's not recommended to not format, since you leave behind the mess from the previous installation then, which means it's likely not going to fix whatever you're trying to fix by reinstalling. In any case, it's only the Windows partition you need to format, not the entire disk. If your entire disk only consists of the Windows partition though, then you better have a way of backing up everything you can't otherwise recover like Unne mentioned :p
b) As for how long it takes, half an hour on a decent system is normal for Windows alone, but then you have to remember you're going to need to reinstall all the other programs you use too, which will take much longer, plus Windows patches too.
c) This is why I dislike activation :p I suppose it's acceptable for normal average PCs users that don't know what they're doing half the time though :p