Access Denied - various Windows O/S

I've searched the forum and tried solutions suggested in previous posts without success.

My setup is this:-

PC 'A' - Windows XP Pro - cable connection to Netgear router
PC 'B' - Windows 7 Pro - wireless connection.
Laptop 'C' - Windows 7 Premium - wireless connection
Laptop 'D' - Windows XP Pro - cable connection
Netbook 'E' - Windows XP Pro - wireless connection

All machines run AVG Pro but do NOT use the AVG Firewall; instead each relies on the Windows firewall. All have access approved on all HDs for all users of the LAN. When logged on to each machine as the same user - i.e. 'me'.

'A' sees and connects fully with all machines
'B' sees all. Access denied to 'C'. 'D' asks for username/password and doesn't accept anything I enter.
'C' sees and connects fully with all machines
'D' sees all. Access denied to 'C'. All others full connection.
'E' sees all. Access denied to 'C'. 'D' asks for username/password and doesn't accept anything I enter.

I can't make head nor tail of this. All machines see each other; two have full two-way connections; three have access denied to 'C'; two are asked for user/password when trying to connect to 'D'!

Would appreciate any ideas for a solution.