Photo stream only keeps photos in iCloud for 30 days (even though they remain on your devices until deleted). If you just recently enabled photo stream on your PC, the 75 photos are the ones added in the last 30 as the older ones are no longer in iCloud.
If they are still on your device, you can import them from the camera roll as explained here: http://support.apple.com/kb/HT4083. If they aren't in the camera roll, save them from your photo stream album by tapping Edit, tap the photos, tap Share, tap Save to Camera Roll. Then they can be imported to your computer.
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