Since it doesn't sound like you're providing any credentials from the Pi, perhaps ensure you are allowing Anonymous access to the Windows shares.
Keep in mind "Everyone" is not the same as Anonymous. But you can make Windows behave that way...
To enable anonymous access on a local workstation or server computer
Open Local Security Settings. (Start->
In the console tree, double-click Local Policies, and then click Security Options.
In the details pane, right-click Network access: Let Everyone permissions apply to anonymous users, and then click Properties.
On the Local Security Settings tab, click Enabled, and then click OK.
Instruction source is Microsoft
Note: I don't think
Secpol.msc exists on Home versions of Widows.