First off, let me say this:
Do not play around with Local Security Policy on your WHS. You'll probaby break your WHS, which will most likely require a Server Reinstallation to fix.
Now, as to your issue: if your MC box is set up right, this really shouldn't be an issue. If the account that you're logging on with has a matching account (same username, same password) on the WHS (and, said account has access to the share(s) you want it to have), it should JustWork.
On a WHS, password policy is controlled in the Console (Settings > Passwords.) The default setting (Medium) is 5+ characters, 'complexity' ("Strong" passwords) not required for local logon (Remote Access, though, always requires a 'strong' password.)
This might be your problem - if you're accessing your WHS with an account that's enabled for Remote Access (but, the MC box isn't using that same account name/password for local logon), then you'll need to enter that 'strong' password for access.
Make sense?
HTH (and, please, feel free to ask questions if needed),
~Chris
'nearly every day of my life is some kind of computer hell'
Desktop: E7300 on GA-P31, 2GB, HVR-1250, HVR-2250, ATI DCT (internal), HD4650, Win7 Pro x86.
Server: Intel E6400, GA-965G, 4GB, 2x 500GB WD, 2x NV DualTV, 3xHDHR (2 QAM, 1 ATSC), HD3650; 2x Xbox360; Win7 Ultimate x64