Better still, for 1:1 briefs why not forget presentation software altogether and stick with paper (for pre-printed diagrams or pictures) and white board. You'll get much more student involvement that way.
If you're doing a lecture to a group it's a different dynamic and presentation software can be useful.
HFD
(BTW, I use "presentation software" because you can get excellent free office software from Sun (OpenOffice) and IBM (Symphony) - you don't have to pay for MS Office)