Custom Data Validation That Evaluates for Numbers AND Text

I am trying to create a custom data validation rule that would allow only specific values into the cells. These values can be only quarter decimal numbers, whole numbers, or certain text. I used the OR function and it seemed to work. However, I get the #VALUE! error when I mix values and text. I am wondering if the problem is that although I have version 2010 at home, I am working on a laptop that only has version 2003 of Excel.

Can someone please help me out?

Here is my formula:
=OR(C17-0.75=TRUNC(C17), C17-0.5=TRUNC(C17), C17-0.25=TRUNC(C17), C17=INT(C17), C17="A", C17="G", C17="T", C17="D")

Thanks a million in advance!
JP

Working with 2003 at the moment is definitely making this difficult! For example, MROUND did not exist
However, I changed that part and tested it and it looks like this is going to work just fine. THANK YOU so very much for the quick response!
Many blessings to you.

Try this:

=IFERROR(MOD(A1,0.25)=0,OR(A1="A",A1="G",A1="T",A1="D"))