Ctrl + C and Ctrl + X
Copy (C) or Cut (X) the highlighted text or selected item. Be careful cutting something because it is gone if you don't paste it right.

Ctrl + V
Paste the text or object that's in the clipboard.

Ctrl + Z and Ctrl + Y
Undo any change. For example, delete the wrong part of a sentence, pressing this will undo it. This can also often be pressed multiple times to undo multiple changes.
Pressing Ctrl + Y would redo the undo.

Ctrl + F
Open the Find in any program. This includes your Internet browser to find text on the current page.

Alt + Tab
Quickly switch between open programs moving forward.
Press Ctrl + Tab to switch between tabs in a program.
Adding the Shift key to Alt + Tab or Ctrl + Tab will move backwards. For example, if you are pressing Alt + Tab and pass the program you want to switch to, press Alt + Shift + Tab to move backwards to that program.
Windows Vista and 7 users can also press the Windows Key + Tab to switch through open programs in a full screenshot of the Window.

Ctrl + S
Save what you're working on without stopping.

Ctrl + P
Print the page you are looking at. This even works in your internet browser.

Alt + F4
Closes the program you are using. If no programs are open it will bring up the shut down menu without you going through the start menu.

Ctrl + W
This closes just the tab that you are in, not the entire program.

Ctrl + T and Ctrl + Shift + T
When browsing the web press Ctrl + T to open a new tab, and if you closed a tab you still want simply press Ctrl + Shift + T to reopen the last closed tab.