I don't want the start menu itself hidden, but I'd like to hide menus and programs in "All programs" that aren't used often. Is this possible?
if you don't mind the good old Classic Start Menu, there is an option to Use Personalized Menus:
which is documented as ".. keeps the Programs menu clean by hiding items you haven't used recently...".
this feature is not available for the regular Windows XP Start Menu, there is no other way than organizing the Start Menu yourself, manually (interactive tutorial) or with a little help from a 3rd party application, such as Winstep's Start Menu Organizer.
Not sure exactly what you need. The taskbar is usually the long one at the bottom with open programs, files and folders. This doesn't have an "All programs" element (by default.) Do you mean the toolbar at the bottom right that shows a bunch of stuff happening in the background?
Guide here. Basically, right click on taskbar, properties, hide inactive icons.
In customize, you can choose features for particular applications.
Probably some of the programs can be disabled from starting automatically.
If you mean the "all programs" menu in the start menu (bottom left), then there'll maybe be another method.