There isn't one assigned in the "out-of-the-box" program, but it's
very simple to create one.
- Open Tools > Customize and click the Keyboard button.
- In the Categories list, click View.
- In the Commands list, scroll down and select ViewZoom100.
- Put the cursor in the "Press new shortcut keys" box.
- Press the key combination you want to use. I'll suggest
Ctrl+Shift+0, which isn't normally assigned to anything.
- Click the Assign button, then close both dialogs.