conditional statement formula

Hey all... this seems to be the best place to get some answers, so here goes...

I'm setting up a sheet for personal financial help, and basically, I've set it up where I can input a value for my paycheck, set up a list of budgeted expenses and bills, and then have a value at the bottom to show what is remaining in my checking acct.

Where I've run into my problem, is I would like to be able to put an "x" (or some other value, I'm not particular) in the cell immediately to the left of the expense when I actually pay it, and have the corresponding expense be taken out of my total account balance... sort of a way to balance my checkbook.

I understand my description is vague, so please ask any questions you need to help clarify! Please cut this noob some slack !

Thanks!

Perhaps something like...

=Sumif(A1:A100,"<>X",C1:C100)

where A1:A100 is the range you mark with an "X" and C1:C100 is the range that gets summed if there is no "X" in A1:A100...