I have problems accessing folders shared on the network on a Win7 machine from a virtual WinXP machine. What can I do to fix it?

This is the situation: I am running a virtual WinXP SP3 Professional in VirtualBox on a Windows 7 host. The XP accesses the network via a bridged network adapter, and I can access the Win7 machine over this connection without a problem. Both systems are in the same Workgroup and get their local IPs assigned by DHCP.
Here is the problem: Some of the shared folders are accessible and I can list folder contents, but I can neither write to, nor copy files from these shared folders, and other shared folders return an “access denied” error message, “even though” the sharing permission settings are identical to the shared folders that I can access.
Sharing permissions are set to full access for “everyone” for all shared folders.