You could create a list if you have excel 2003 or higher. It is pretty simple to do what you want. Create a new sheet, and then select cell A1. From the menus choose Data>List>Create List. In the first column you will put your x & y values. In the second column you will put the a,b,c values. In the third column you will put the d,e,f values. Now you will need to define a name for each column. ***NOTE*** To avoid unwanted blanks in your pull downs it is best to skip a column every time you create a new list. So the list I have described should be in column A,C,E. In the first column can give it any name you like, so lets just name it product. To do this select all of the cells in the first column, that are within the list, all the way down to the *. At the top of your screen, next to the formula bar you will see the "name box". It's the box that shows the address of the cell you have selected. When you huver over it a hint should pop up saying "Name Box". Click within this box, and type in product. Now press the Enter key. You can use any name you like, but spaces are not allowed. Instead use an underscore _ . The other two columns do have to have a particular name. You will perform all of the previous steps for naming the other columns except this time you will name the second column X, and the third column Y.
Now to actually create the validation list... Go to the sheet you wish to have the drop downs in. Select the cell where the user will choose x or y. From the menus choose Data>Validation then select the Settings tab. From the allow menu choose List. In the source box type =product, and press OK. Now in the next column over, where the attribute are chosen, create another validation list just like we did before except this time instead of =product you are going to type =indirect(A1). Replace A1 with the actual cell address of your first validation list (The cell with the X,Y choice). The great thing about this is that you can add to these list, and the new items will automatically be added to the drop down list. If you have a new list of attribute (g,h,i) later on then just use this as a guide to create an additional list. Also keep in mind that you can not name a range A345 or anything that is a cell reference. You can use A_345 though. HTH
If I did not give you a good enough explanation then you could probably Google: "dependent validation list" for a different explanation.
