If the log is on the same drive, don't purge any data until you've given the log more space!
If the log is out of space, and particularly if it getting errors about it, you must add log space first. If necessary, you can temporarily add another log file on a different drive. First you must get the log file in working shape, then and only then can you safely purge data.
SQL DBA,SQL Server MVP(07, 08, 09) Prosecutor James Blackburn, in closing argument in the Fatal Vision murders trial:
If in the future, you should cry a tear, cry one for them [the murder victims]. If in the future, you should say a prayer, say one for them. And if in the future, you should light a candle, light one for them.