Use the following formula to accomplish that:
range is of course the one you set it to be !!
EDIT 29-03-2013; 23:00 CEST
IMPORTANT: This is only valid if you work at least 70 hours per week, each week. It is just as important to know if you have worked less !! Therefore you need to change the way you calculate the "overtime". The following formula will show only a calculated value, if a "hours worked" value has been entered:
See example file, column E.
EDIT 29-03-2013; 22:30 CEST
I've created an example file: Excluding Negative Values. Here I tried to clarify the problem, the way the OP sees it. In doing so, I incorporated the other answers as well.
The solution given by SpreadsheetPro.net works perfectly. The statement about the
SUMIF(S) not being available in Google Spreadsheet, is false.
The solution handed by Al, covers a different approach. Instead of calculating
Hours Worked MINUS Hours Working Week, Al is telling the formula to return
ZERO is value is below
ZERO, see column D in example.