Do you use WEP encryption?
Maybe I will answer to your question using google results. When you get to the last page of results for
- "This network is marked as a hidden" (you will find 3 pages - 24 results)
- "This network is marked as a hidden" wep (2 pages - 14 results)
Do you see some correlation? Majority of results include information about WEP. Some of topics were never answered. Those one were the solution was found include suggestion about changing WEP encryption.
Try to set another security type in your router's settings and use it in your network setting under Windows 7. I'm using security type: WPA2, and encryption type: AES and everything works fine.