There are many ways of doing this (and I encourage you to look them up as they will be more efficient generally) but the simplest way of doing this is to use a non-set operation to define the value of the third column:
SELECT
t1.previous
,t1.present
,(t1.present - t1.previous) as difference
FROM #TEMP1 t1
Note, this style of selection is considered bad practice because it requires the query plan to reselect the value of the first two columns to logically determine the third (a violation of set theory that SQL is based on). Though it is more complicated, if you plan on using this to evaluate more than the values you listed in your example, I would investigate using an APPLY clause. http://technet.microsoft.com/en-us/library/ms175156(v=sql.105).aspx