If the # of rows you want to delete is much larger than the # of rows being retained, you might want to do this....

- Copy the rows you want to retain to another table
- TRUNCATE the original table
- Load the table you created in step 1 back to the original table (or rename it, or whatever)

Obviously, if you have constraints involved, you'll need to disable or drop them and then re-enable-re-add them

Just a thought