I have the same issue, but it occurred after I followed the Synology tutorial on securing your server on the internet. I believe that after I created a new admin user and disabled the default admin user (as the tutorial suggested doing), my mapped diskstation drive in Win7 stopped working. After re-enabling it, the connections to the mapped drives resumed.
So my question is, how to I change it so that Windows look for my new username and password when mapping the DS?
Update: Ok, so I ran 'control keymgr.dll' in Windows and sorted out the password issue and my mapped network worked once again.