How to draw truly vertical or horizontal lines in PowerPoint

Double click on the line and change the height to 0 in the "size" window (near the top right)

Note: if the height doesn't change, click on the down-arrow next to the height to force it to zero.

Note2: Sometimes for almost-horizontal lines, the height will say it is zero, even though it is not quite zero. Click the down arrow and it will make it truly zero.

Hold down the shift key while drawing the line. In 2007 and probably 2010 you have to hold both the CTRL and the Shift keys down.

Also, if it is 2007 be sure and download SP2, there appears to have been a problem in SP1 which makes it impossible to draw a straight line (for some).

The best solution is the line properties making 'size' zero width for verticals and zero height for horizontals. The Ctrl/Shift still not working.