If your not using permission based sharing, Try this...
Open My Computer, Tools, Folder Options, View Tab, At the bottom of the Advanced Settings scroll box, Check Use Simple File Sharing, click o.k. Now right click on your folder you want to use, select properties and select the sharing
Your network admin may have sharing turned off for clients. Depending on your networks complexity, you probably have permission based sharing. To share based using Permissions you'll want to consult your network admin so you don't get in trouble for something. They can be real whiny
"If you share a folder you'll get the dooms day virus and the universe will implode!" Usually something to that extent.
This article discusses both simple sharing and advanced sharing (network permission based) to get you sharing.