Quality of font rendering varies greatly depending on platform and application. Mac OSX uses a generic rendering algorithm which displays all fonts equally well and completely ignores font hinting. Windows on the other hand only displays well TrueType fonts which have been optimized (hinted) for the screen. Most problems are with very old versions of Windows 95, where ClearType rasterizer is turned OFF.
We offer two different kinds of fonts Screen fonts extensively hinted for the best results in small sizes for Windows, resulting in sharp legible text
All other TPTQ Arabic fonts which are simply auto-hinted. Those will work just fine in all operating systems, but if majority of your users use old Windows, than we recommend that you use it in display sizes only (16pt higher). In either case, we recommend that you take the free webfont trial licence, so you can test the fonts in the real situation.
For more details about hinting and screen optimisation, please have a look at this explanation.