Restricting entry

[scubas]

Hi, I have a simple spreadsheet which I need help with. I need to prevent a
customer from using more then one computer (C*) and or laptop. What I need
is a way of stopping more then one yes per row. I have validated the cells
with a list of Yes/No, but cannot find a way of stopping the Yes being
entered if a yes has been entered in a different cell on the same row?

Own Laptop C1 C2 C3 C4 C5
Customer 1 Yes
Customer 2 Yes
Customer 3 Yes
Customer 4 Yes
Customer 5
Customer 6 Yes
Customer 7
Customer 8 Yes
Customer 9 Yes
Customer 10

Any help with be appreciated.