Sometimes you want to assign your newly computed variable a system missing value (which is a . in spss database). Here is the command for that:
COMPUTE temp = $sysmis.
(this syntax will create a variable called temp which will initially have all values set as missing)
For a conditional function:
IF sysmis(v1) v2=$sysmis.
You can also use missing values in RECODE.
RECODE [your command] (ELSE=sysmis).
Also read the UCLA SPSS page and CDC page on handling of missing data.