actualy, if you access the share by network drive the 2 pcs don't have to be in the same workgroup.
you should make sure you have an acount in the pc you're accessing - that is if guest is disabled and all the other acounts have passwords. by default, when you create a share, the group Everyone has only read permission, so if you want to write in the shared directory, give the acount you're using change/full control. Also, if the file system is NTFS, check the security permissions.