In 2007:
1. Highlight all the cells on the rows you want formatted
2. Choose Home, Conditional Formatting, New Rule
3. Choose Format all cells based on their values
4. Choose Icon Sets from the Format Style drop-down
5. Choose 3 Symbols (Circled) from the Icon Style list
6. Choose >= and enter Value 1 and Type Number
7. for the second choice pick >= 0 for the value and Number for the Type.
7.5 if you want turn on the Show icon only option
8. Click OK twice
This is the best you can do because there is no unchecked box graphics in
the conditional formatting area
Now enter 1 for completed tasks and -1 for non completed tasks. You can
design a formula to calculate one of these values.