In Photoshop, if Image Previews is set to Always Save (under File Handling in Preferences), then Photoshop will generate a composite image of your document when it is saved. This is used to show a preview by OS X in QuickLook and other apps. It's quite likely Windows 7 is the same as OS X, and will require Image Previews to view the previews in Explorer.
However, if Image Previews is set to Never Save, then your PSDs will not contain previews and they won’t be able to be viewed by QuickLook, Preview, TextEdit or other OS X apps.
For the setting to take effect, you may have to save your document to a new location with Save As, rather than just using Save.
Saving image previews will make your PSDs bigger. Choose wisely.