Windows 7 - Sharing / Permissions Win 7

Hello, all, I've shared the root c:\ on my main machine with full permissions for everyone, also in the same machine I have a drive named Backup, which I've also shared with full permissions.
When I try to access c:\ from any of my two laptops access is granted, from my Dell Vostro on first access (to my main machine) I am asked for username & password when supplied access is granted to c:\ and Backup shares.

On my Dell Inspiron on first access (to my main machine) no username & password are required and I can access drive c:\, however access to Backup drive is denied with the message you do not have permissions to access this drive...contact your administrator.

I have un-shared the backup drive and restarted the main machine and re-shared it to no avail. Any other ideas???

all machines using Windows 7 ultimate x64

Thanks....