Using the original Dell CD that came with the machine, you will not be asked specifically for the key. The Product Key is always on the system chassis.  On the 8300, check the left-far-rear corner of the box.  Wherever it is, it will be a small white/colored sticker on the computer case.  
I have a Dimension 8300 and need to do a restore to Dell image but I cannot locate an XP Pro Product Key on any of my packaging/CDs! Firstly, do I need the XP Pro product key for a Dell image restore? Secondly, if I do then any advice for where I can locate this Key - it's not on the XP Pro CD sleeve, nor on/in the XP Pro booklet. Thanks all... 
Product Key for XP Pro?