Yes, this can be done using variables and advanced actions, have explained the work flow several times to different users.

Try first to answer a couple of questions:

1. Do you want to enable the user to click multiple times on the same click box?
2. Can the user return to this slide, so that you'll have to construct a 'Reset' scenario?
3. Did you already use advanced actions and variables?

The reason for the 1st question is that if you do not want to enable multiple clicks, you could work with a simple counter and the action triggered by clicking on a click box could increment the counter, and disable that click box, while testing if the counter has reached the number 8. Only when this test results in true, you should enable the last click box.

But if the user can click multiple times on the same click box this simple scenario is not possible, you'll need more variables to store which click box has already 'suffered' a click. Hope this makes sense.

Second question: if the user can return and repeat everything, you'll have to reset the variables.

Lilybiri

Since you are not yet familiar with advanced actions (and variables?), will try to explain in detail and provide some links to articles/blog posts as well.

1. Create 8 user variables, one for each click box (Project, Variables); I will label them v_one, v_two.....v_eight It is not necessary to give a default value
2. Create another user variable v_counter that will be incremented whenever a click box has been clicked for the first time
3. Create a first click box (set to visible), label it CB_One (find that easier) and in the Properties panel, Action accordion, On Success change the action to 'Execute Advanced actions'. Then open the actions dialog box with the Folder icon that appears.
4. You have to create a conditional action, so select that type, and label it CB_One (is possible, an exception because normally you cannot have same labels); this action will need two decisions (have a look at my video tutorial: http://kb2.adobe.com/community/publishing/858/cpsid_85847.html):
   1. First decision 'Visited' checks if the click box has already been clicked, visible when the value of v_one = 1; if this is not the case (first click) v_one gets the value=1 and the counter v_counter is incremented
      
   2. Second decision 'Check' checks the value of the counter, and shows an initially invisible button (I choose a button, BtNext but you can also have a click box) if the counter reaches 8, bummer the forum doesn't allow me to insert an image, have to type:
      IF section                       v_counter is equal to 8
      Actions section             Show BtNext
5. Duplicate the click box 7 times, label them CB_Two,  ..... CB_Eight  (beware: case sensitivity). To each copy the same advanced action is attached, but you'll have to create duplicate actions for each one CB_Two....  In the duplicates you have to replace the variable v_one by the proper variable (v_two...) in the first decision; the second decision remains the same

Test.

Lilybiri