I am typing a document and have apparently hit a key that is causing underlines, strikethroughs, and red letters when I type. What did I do, and how to undo it? I am using word 2007 on an older XP OS.

It sounds as though you've turned on the "Track Changes" feature. Ctrl+Shift+e turns toggles the feature on and off.

Right-click on the status bar and select Track Changes. This adds Track Changes to the status bar. Now, at a glance, you can see if the feature is on or off. You can also turn the feature on and off by clicking Track Changes on the status bar.

As for your document -- make a copy as a backup (very important).
Now, if you want to remove the changes that were "tracked" (the underlines, strikethrough, etc.) follow these steps:
1. Click the the Review tab.
2. Click the Changes group.
3. You can Reject or Accept each change individually --
or you can click the drop down and Reject All Changes or Accept All Changes.