If you are using an activex checkbox (from the controls toolbar In XLXP/2003
or from developer/insert/ActiveX Controls on the ribbon in 2007)
right click it to see properties.
Look for the property Linked Cell.
Link this to a blank cell  in your spreadsheet
Put the formula =cl2=1 in the blank cell
Repeat this for each checkbox using a different cell for each checkbox

You will have a group of cells which have the value true or false - each
cell drives one  checkbox.