Most people I know use DBAN or "Darik's Boot and Nuke". Its a selfcontained bootable utility that allows for wiping Hard Drives. Unfortunately the realities of the technology means you will need to wait a while - DBAN allows you to select how you wipe the drives (how many times and what pattern), a single pass 0 wipe which is enough for 99% of everyone (recoverable only with very expensive technology) will take a little over 4 hours. Even fast hard drives have a sustainable write rate of ~ 100MB/s which means that a 1.5TB will take that long.
DBAN allows for more exotic wiping patterns, like a seven run random pattern which will take almost a day at these transfer rates. Booting into a linux live-cd and running
dd if=/dev/zero of=/dev/sda
would accomplish the same thing as single-pass 0-wipe
Doing a simple format on the Hard Drive will clear the Partition table and takes several seconds, However the DATA is still intact and recoverable easily using standard tools.
If you just want to clear the hardrive so you can reinstall the system a simple format will work fine. If you want to make sure the DATA is "destroyed" a single wipe through is your best bet.
As for clearing specific folders your best bet would be to overwrite the files with random data. Simply deleting the files will free their blocks on the filesystem but not wipe the data. Simple unix utilities like
dd will let you overwrite files so they are unrecoverable.