Need Help To Calculate A Running GPA In Access!

Hello, I have volunteered to do a database for the school of nursing to use at a small Christian University using Microsoft Access 2003. I have Students, and each of these students is going to take each of these 12 nursing courses. Each nursing course has a specific number of credit hours (2,3,6, etc.) and they are to receive a grade for each class (on a 4 point scale, so their grades are going to be a number 0 for F through 4 for an A). I need to have access calculate a running GPA which only takes into consideration the classes they have completed thus far. Any help would be appreciated. I can do it in excel by just leaving the row for untaken courses blank, then autosumming the credit hours column for total hours taken. I can use a formula for each row that just multiplies the grade (like 4.0) by the credit hours (like 3) and get a total grade points of 12 for that course. Then I can add those numbers up(autosum the total grade points column) and devide by the calculated autosum number I would call total hours. I hope it is clear what I am trying to do and why I don't know how to do it in Access. ***************Extra Info*********
I have a student table and each student has a unique studentID, I have a second table called Nursing grades which I have a Student ID field, and 2 fields for each class a grade recieved and a credit hours field (ex. I have PharmacologyGR and PharmacologyCH, NursingResearchGR and NursingResearchCH). If I need to redesign how I have my tables I can do that, I have a unique Course ID for each course I could put in if needed. Basically, I'll do whatever it takes to get this to work.
Thank you so much for even reading this far, and any help would be appreciated more than you know.