First off, you need to get an understanding of your credentials.
1. You have user credentials on your workstation.
2. You have user credentials in Samba.
3. You have user credentials in FreeBSD.
1, 2, and 3 may or may not have the same username and or password.
For ease of use, the username for 1, 2, and 3 should be the same (and not be, on the FreeBSD server, the root user or have a default group of root, admin, or wheel).
Make sure all files in the shared Samba volume that is being mounted on the XP workstation have ownership of your user and have read/write for your user.
Using "Run As" in XP isn't going to make a difference. You can "Run As" the local administrator account in XP, and it won't override the FreeBSD ACLs or the Samba ACLs (unless for some reason, the local administrator username on your XP workstation is valid on the FreeBSD server and Samba service).
The simplest way to get going:
Assuming your XP workstation username is 'bob' with password 'canon', make sure you have an account on the FreeBSD server (/etc/passwd, /etc/shadow) with the username 'bob' and password 'canon'. Create the Samba user with username 'bob' and password 'canon'. Make sure the shared directory (mapped via Samba) is owned by user 'bob'.
Hopefully you have a stronger password than 'canon'.