This might be close to what you want. This will apply a border to every
second line from column A to Column I (change the 1 and 9 to the appropriate
columns you want). If you want to box every 3rd line (rows 3,6,9,12, etc.),
change the 2 to 3.
Highlight the whole spreadsheet. Select Conditional Formating by clicking
on Format->Conditional Formating. Select "formula is" and input this
formula...
=(MOD(ROW(),2)=0)*(COLUMN()=1)
Click on format and select the border tab. Select the left, top, and bottom
lines. Ckick okay. Click Add and input this formula in Condition 2
=(MOD(ROW(),2)=0)*(COLUMN()=9)
Select the right, top and bottom lines. Input this formula in Condition 3
=(MOD(ROW(),2)=0)*(COLUMN()<=9)
Select the top and bottom lines.