That's great, you can use the "new" (as of SQL 2012) and easy approach. I don't have useable sample data to test it with, but something like this should work:
SELECT
Yr_Mnt, Part,
SUM(Part) OVER(ORDER BY Yr_Mnt ROWS UNBOUNDED PRECEDING) AS Total
FROM dbo.Part_Tbl
ORDER BY Yr_Mnt