Data Entry in Excel 2000

I prepared a data entry form on Excel that have four fields:
Annual Compensation Rate:
Hourly Comp. Rate:
Change Amount:
Change Percent:

I need that when the user enters data in either one of those fields the rest be automatically computed without overwritting formulas. For example:

If the user enters 10.00 in Hourly Comp. Rate, the Annual, Change Amount and % be calculated. Or he changes his mind and enters 10000 in Annual Comp. Rate the rest be computed without overwriting the formulas.

Thanks,

Wgonzlez.