Sum of current row from selected collums

Tried looking this up, and could only find how to do parts, but not how to do the complete function.

I want to have each cell in one column, say, column O, to have the sum of that row, but only from selected columns, so lets say columns B to N.

I know how to make a function into each row to get the sum of that row using =SUM(B1:P1), =SUM(B2:P2), etc. But I want to make a function that will do that so that it is dynamically applied to that whole column. This way I won't have to create the function for each row. It's already there, and I just need to put the data in.


I tried looking it up online, the best I got was this:
http://www.expresscomputeronline.com...chspace1.shtml
Maybe I just don't understand, but that didn't get me anywhere

So anyone know what to do?
Oh yea, I'm using Excel 2003.