Text is messing me around!

I have never had this problem before and I suppose it was only a matter of time. I designed a site for a client (www.availabed.co.za) and took a lot of trouble to make it look like she wanted it. Then the problems started. On her two different computers the text appeared different: On the one it was too small, and on the other it was so large that it changed the size of the layout region and messed up the look of the site. I tried using relative size (-1) format for text size, pt size and also pixel size and nothing made a difference. I have evntually resorted to taking a screenshot of the site and the whole thing is now graphics-based which is of course not ideal. I designed it in Netobjects Fusion 7. Any ideas how I can sort this out?