Windows 7: [Ultimate] How do you gant another LAN user permissions?

I've upgraded my home network file server to Windows 7 from XP.
The server houses all the media and photo files.
When it ran on XP, I had no trouble adding/deleting files on the serer from another computer on the same network.
But with the upgrade, I can read but cannot write or delete files on the server from another computer.
I've tried the file Properties - Security tab, but there seems to be no way to add a user from another computer.
Hours of search on the internet hasn't helped either.

Can someone enlighten me?

Hi! Take ownership of the folder

Go into Network Connection and Sharing.
On the lift side click on Change Advanced Sharing settings
Under Home/Work Turn on the Following
- Network Discovery
- File and Print Sharing
-Sharing so that anyone with network access......
-Media Streaming

Turn Off Password Protected sharing