Not sarcasm. You're loosing connection. There's a limited number of things to try.
Windows is kind of stupid when it comes to this sort of thing. It says it's losing connection, but it's really losing the device logically.
Never Retreat: I read through the manual. This thing appears to run some sort of SMB Share (prolly Linux with Samba). I suspect you're having a workgroup/domain issue. I've run into similar problems with Linux/Samba myself. The error would be similar or identical to what you posted above.
Check your workgroup settings. If you hardcoded an IP, put that into your hosts file for good measure.
It supports FTP access. Use that as a test. FTP is not dependent on Microsoft's SMB networking, so it'll work even when "share level" access fails. If that fails too, then you likely have a connectivity issue, but if it works and your normal access method fails, you definitely have an SMB/workgroup/domain issue.
On page 24 of the manual, there is a section on setting up permissions. One of the tricky things about workgroup level networking and permissions is that authentication is managed by each host. Even though you *think* you are using the right account, there could be minor differences that make it not so. Make sure usernames and passwords are spelled correctly. Make sure the account on the NAS matches what you use to log into your computer (that info gets passed to the NAS when you access the share). Sure, you can use different accounts and provide those credentials when you access the share, but it adds another failure point.
Do as AJ says and make sure your permissions are populated from top to bottom and allow your user account full access. I don't think this is the problem since it affects your ability to access directories that you were in recently, but it wouldn't hurt.
Unfortunately, this thing doesn't appear to support WINS and that is a big help with Workgroup level networking. I know MS did away with it recently, but IIRC, it is still there in XP.
Chris