Now that the installed base of Macs is growing, especially outside the large enterprises, you need to take into account that your PowerPoint presentation is likely to be opened on both machines.
There are obvious differences to be aware of. The key one is fonts: there is a large set of fonts that are available on both operating systems, but very obvious ones are not always part of the overlap (Helvetica for example is not available on a standard Windwos machine, and Calibri gets only installed on a Mac once the user buys Microsoft Office).
But here are the less obvious ones. Even if you stick to standard fonts, there are still tiny differences in how both operating systems insert line breaks. Watch out especially for tight text in boxes.
Also, there is an annoying difference in the way PowerPoint for Mac colors text and shapes. You pick the same colour for both, but they look different. A design can look perfect on a Windows machine, but off on a Mac.
There is no quick solution to all of this. Installing a second virtual machine on your computer might be a bit overkill. I guess there is no alternative but to ask a friend or the recipient of the presentation to send back a quick PDF file to double check, especially for important presentations that will be presented on screen (as opposed to a document meant for reading).