keyboard shortcut in Windows 7 to center window

You should check out WinSplit Revolution; it has what you need and is customizable.

This is not exactly for centering, but lets you move the window left and right (and up and down) easily.

1. Focus a window.
2. Press Alt+Space.
3. Press M (for “Move”).
4. Use the arrow keys to move the window exactly where you want it.
5. Press Enter when done.

The Windows+arrow keys are quite useful here.

• (Windows)+← (Left) and +→ (Right) cycle through going to the left or right 50% of the screen, and restoring the window to its original size and position.
• +↑ (Up) maximizes the window.
• +↓ (Down) restores the window if it is maximized, and minimizes it otherwise.