I’m not sure which steps you were following, so to start off, make sure that you’ve zipped your projects before mailing them to yourself. Although they appear as a single file on the Mac, projects are actually folders of files saved in a package format; they won’t send properly unless they are zipped into a single file. To do that either choose File > Back Up > Back Up To… from within the project to create a zipped backup that you can attach in your email or Ctrl-click the project’s .scriv file and choose the “Compress” option to create a zipped copy.
On Windows, you should then be able to download the zipped project, right-click it and choose “Extract All…” and then save the project to whatever location you like and load it from there in Scrivener as normal. However, occasionally you’ll find that Mac-zipped files trip a permissions flag on Windows and the files will extract with some of them encrypted. Usually the filenames will appear in green text in the file browser to indicate they’re encrypted. In this case often the project will open but none of the files will load.
Try closing the project in Scrivener, then right-clicking the extracted .scriv folder and choosing Properties, then Advanced (on the “General” tab, which should be what’s visible when the properties opens). In the Advanced window, deselect the option to “Encrypt contents to secure data”, then OK out of the advanced attributes window and click “Apply”. You should get an option to apply the changes only to the selected file or to all the folder’s subfolders and files as well; choose that latter. Ideally Windows will decrypt everything and you can click OK to exit the Properties window. Then try opening the project in Scrivener.
Finally, if the unzipped project is not loading at all, open the folder and take a look for multiple copies of the yellow-icon .scrivx file (it will probably be named for the project, i.e. if your .scriv file is called “MyNovel”, then inside the .scriv folder on Windows you’ll find a “MyNovel” .scrivx file with a yellow Scrivener icon). Multiple copies can prevent the project opening on Windows, so if you see this, remove all but your most recent and then try again. If that works, you may want to extract another copy of the project–one for each of the .scrivx files you had to remove–then repeat the process but leaving a different .scrivx file in the project folder each time, so you get a separate copy of the project folder for every version. (Give the projects distinct names after you extract them.) You can then open those all side by side in Scrivener and compare them, in case there are conflicts you need to resolve–e.g. files appearing in one that are not shown in another. You can pick one as the master, then drag any missing files from the binder of the other projects into the master’s binder. When you’re finished, close them all in Scrivener and trash all the copies but the master.
I’m hopeful some of those solutions will solve the problem, but if you’re still not having success, could you detail exactly what steps you’re taking to send the project to yourself and open it on Windows, and also which version of Scrivener you’re using on each and which version of the Mac/Windows OS?