My favorite keystrokes ranked in order

1) Ctrl C - copy
2) Ctrl V - paste
3) Print Screen - puts an image of what you are viewing on your screen onto your clipboard
4) Windows key + M - minimize all windows
5) Ctrl + Alt + Delete - in Windows XP it brings up options to restart computer and close programs that are not responding
6) Ctrl X - cut
7) Ctrl A - highlight all
8) Ctrl Z - undo
9) Ctrl Y - redo
10) Ctrl O - open
11) Ctrl P - print
12) Ctrl S - save
13 Alt-Tab - flip between windows