I assume you mean horizontal or vertical lines as opposed to just straight as that is what the shift key does in ID. If you're not using a mouse, try plugging one in to see of it could be a trackpad problem - usually caused by moisture. Also make sure the problem exists in a new blank doc - to rule out some sort of wierd corruption of the document that it's happening on
Beyond that, the I can't think of of any reason why the problem would exist. Usually trashing prefs fixes a lot of things without hurting anything so you might try that. Quit ID & go to User/Library/Preferences/Adobe Indesign. Trash the file named Indesign Defaults - and NOTHING else. Start up ID and see if the problem went away.
You might also check to see if the same problem happens in Illustrator or Appleworks.