Ask yourself the following question: Would it be bad if someone were to INSERT a new row into your data while your transaction is running? Would this interfere with your results in an unacceptable way? If so, use the SERIALIZABLE level.
From MSDN regarding SET TRANSACTION ISOLATION LEVEL:
SERIALIZABLE
Places a range lock on the data set,
preventing other users from updating
or inserting rows into the data set
until the transaction is complete.
This is the most restrictive of the
four isolation levels. Because
concurrency is lower, use this option
only when necessary. This option has
the same effect as setting HOLDLOCK on
all tables in all SELECT statements in
a transaction.
So your transaction maintains all locks throughout its lifetime– even those normally discarded after use. This makes it appear that all transactions are running one at a time, hence the name SERIALIZABLE. Note from Wikipedia regarding isolation levels:
SERIALIZABLE
This isolation level specifies that
all transactions occur in a completely
isolated fashion; i.e., as if all
transactions in the system had
executed serially, one after the
other. The DBMS may execute two or
more transactions at the same time
only if the illusion of serial
execution can be maintained.