Since hard disks have an integrated electronics (that is, for 20 years) actual low-level formatting is done at factory (except for some operations on Scsi disks). What is called low-level formatting only means wiping all sectors - including the partition table, boot sectors etc - and testing them all.
So it's a lengthy but rather efficient way to erase private data, boot sector virus, C-Dilla data and a few more. Can be good if you believe your OS inadequately marks sectors as bad because of its software errors, or because the previous host was faulty (Via for instance). Nice also before selling your used disk or using one bought used.
Windows or Ms-Dos applications exist to do it in a better targeted way, but sometimes a plain, reliable full erase is just preferred.
For that operation, I would not use a software of unknown origin running on Windows. I'd use the free software provided by the disk manufacturer which runs on its own Dos.
^ Agreed. Do NOT try to do a true "Low-Level Format". In fact, you probably cannot get any tools to do this, despite the claims. Pointertoavoid is right, it probably is just a disk wiping tool. That is useful only if you really MUST destroy data on a disk.
If all you need to do is start with a "blank disk", the simplest way is to Delete all its Partitions, then start by Creating one or more new Partitions and Formatting them. These operations will NOT wipe out all the old data. But they will completely wipe out and replace all of the systems for tracking the use of sectors in files so that no file can be found again without fancy tools. If that's all you need, do the simple steps.
If you believe there are bad sectors on the HDD, your best starting point is disk utilities downloaded for free from your HDD manufacturer's website. They understand all the details of their drives and can handle them properly, both in terms of diagnosing problems, and in terms of finding and fixing minor errors.
At the time a HDD is manufactured and the REAL Low-Level Format is done, the controller board mounted on the HDD unit stores a bunch of information on which parts of the HDD it is using, which sectors if any are faulty and marked "Never Use", and which sectors are good but unnecessary for now, and hence are held in reserve as spares for automatic error fixing. As the disk is used, one of the background functions this board runs (even unknown to Windows) is to monitor all reads and writes for advanced indications of weak signals and bit errors. When it detects a poor sector it will copy the data from it to a known-good spare sector, mark the bad one as "never use again", and substitute the good spare in its place. If this all works, the data is preserved long before the sector is so bad the data are damaged. Windows will never know there was a "bad sector". As this work is done the HDD's board keeps track of it all and of the stock of good spares. One of the functions in the S.M.A.R.T. system is to warn you ahead of time if the stock of good spares is running low. If you get such a message, replace the drive soon while the automatic repair system is still working!
One function that the HDD maker's utilities can do is to force this internal error checking and sector replacement system to go through the entire disk and replace any questionable sectors it might have missed. It's a long process, but should be one of the first "fixes" you try if you suspect a problem, because it CAN actually recover data from a questionable sector before it fails completely and causes data loss. Note that this does not really fix a bad sector. It replaces it with a known-good one, preserving the data if possible, but Windows does not know this detail.
Also note that any software that really tried to do something like a Low-Level Format would severely interfere with this whole process, because it could fail to provide correct information to the HDD board's system, leaving it misinformed about which sectors are good, bad, and good reserves. SCARY!
If you run the disk maker's utility package and use all its diagnostics until the unit appears to have no problems, then is the time to let the OS start using it. As I said, if you're trying to re-use a disk and want to start empty, first step will be to Delete all its Partitions and then Create and Format one or more new Partitions. Then you can run Windows' Chkdsk utility to make sure Windows is happy that the whole disk (at least, the part that the HDD tells it about) is good. In fact, the easiest way to do that is to use the Full Format version during formatting. That will have Windows do its own checks on every sector available.
If you are planning on using the HDD in the new system, you don't even need to do a full format really. If you are giving it away or selling it, you'll want to make sure your data is destroyed properly. Something like DBAN will do that for you nicely.
If you going to re-use this drive in a new system, the OS will format and partition the drive when you start your new install. You don't need to do anything at all to it before hand. Unless, like randomizer has pointed out, you are giving the disk to someone else.
If you are doing a complete new install and don't want to preserve anything on the HDD (maybe you already backed it up to another device) don't bother Formatting or anything ahead of time, as others have said. BUT when you do start the new install, look closely at the Install process options and find where you can Delete any and all previously-existing Partitions. You don't want any old stuff hanging around and using up space!