that you have assigned the same workgroup name to both computers on the network? You say you've checked this.
This message sometimes appears when nothing is wrong, and often a repeat request works. If that isn't the case on your network you need to check several things.
1. That the desktop firewall isn't stopping your access.
2. That the correct permissions are set on the desktop - right click the folder or folders that are shared and select properties. Click the 'Sharing' tab and then click the permissions button.
3. Make sure that the group 'everyone' has all three permissions ticked - full control, change and read.