If formula to replace the #N/A value

[Casper]

Hi, I have been trying to work out the correct way of creating an if formula
to no success, it's probably not that difficult but hope you can help

I have a few columns with info that has got vlookup formulas in them and
some of the cells will have a #N/A value but will have the correct values
after a few days of data entering, I am trying to create the if formula so
that where there is a #N/A value to replace it with anoter cells value for
example;

A B C
Credit Value Date Total
1 12/01/2007 1
2 #N/A 0

if there is a date in the date column(B) then I want the value of column A
to be placed in clumn C but if there is a #N/A in column B the value in C
must be 0

Please help
Regards
Casper