Insert a column with the formula =RAND() for each row
This generates a random number between 0 and 1 for every
row it is copied to. It will update whenever the
worksheet is re-calculated, so if you sort by this new
column, you can pick off the top 100, or create a new
worksheet and reference the first 100 rows.
Without sorting, you can do this. Enter =RAND()<1/8 in cell H2, for example.
Select a cell in your range of records, use Data/Filter/Advanced Filter,
select H1:H2 as your criteria, and you'll get APPROCIMATELY 1/8th of your
records (100/800).