Send me an Excel worksheet that shows how to determine the statistical
significance of the following scenario:  In Week 1, 100 customers
enter Carmen's Candy Store and 5 of them purchase a Luscious Pops that
week.  In Week 2, Carmen moves the luscious pops display from the back
wall onto the counter.  This week 105 customers enter the store and 15
customers buy a Luscious Pop.  Did moving the candy display have a
statistically significant effect on the sales?  To answer the question
I'm looking for an Excel worksheet I can use to run these kinds of
questions again using different input values.

I'm not a researcher, but here goes:

week	# cust	# purch	# non-purchasers
1	100	5	95
2	105	15	90

		pval:	6.74877E-06

Note that the only formulae are in the last column (column D): 
D2=B2-C2, D3=B3-C2, D5=CHITEST(C3:D3,C2:D2).  This will provide a
general solution to your problem (just enter the number of customers
and purchasers for each week in columns B and C), but beware: if any
of your cell entries fall below 5, CHITEST() may no longer be an
appropriate test statistic.

You cannot determine statistical significance from two measurements.

By definition two measurements are lineked by a straight line.

If you consider the customers to be independent of eachother and it is
believed that the only thing influencing the number of pops sold was
the placement of the pops, then each week will be like a binomial
distribution (and hence the variances can be estimated independently).
More importantly, useful data then can be extracted. (If there was
just one person in each week, then it would be a problem, but even
then there are ways to work with the data... just not very powerful
ones.)

wethepeople, are you always looking to have just two types of states?
(For example, purchasers and non-purchasers.)

I will be brutally honest since the question sounds rather request for
service and does not "Answer". Hire someone, unless you are so
unfortunate. Moreover, Google Answers do not support attachments and
writing formulaes here cannot guarrantee that you can find the
function to begin with (otherwise you would have done yourself!) So I
will just write down, with similar table as previous commentator. And
you request only the formluas, I won't explain.
-Table-
week  cust               purch
1     100                5%
2     105                14%
mean  AVERAGE(cust:cust) AVERAGE(purch:purch)
sd    STDEV(cust:cust)   STDEV(purch:purch)
p                        ([2purch]-[meanpurch])/[sdpurch]

I will be brutally honest since the question sounds rather request for
service and does not "Answer". Hire someone, unless you are so
unfortunate. Moreover, Google Answers do not support attachments and
writing formulaes here cannot guarrantee that you can find the
function to begin with (otherwise you would have done yourself!) So I
will just write down, with similar table as previous commentator. And
you request only the formluas, I won't explain.
-Table-
week  cust                purch
------------------------------------------------------------------------
1     100                 5%
2     105                 14%
mean                      =AVERAGE(purch:purch)
sd                        =STDEV(purch:purch)
pval (this is correction) =1-NORMSDIST(([2purch]-[meanpurch])/[sdpurch])