I'd like to add a couple comments to those made about TASERS and OC:
OC - I've been teaching police chemical agents for right at 30 years -
When OC was introduced to the LE world as the replacement for the officer's Chemical Mace (CN) one of the things that initially gained favor was the fact that the OC particles must have contact with the face, eyes, respiratory tract to be effective, unlike the chemical agents CN and CS which have very small particulates, low vapor pressures and therefore spread easily and effect everyone in the immediate area.
With the OC projectors (sprays) one of the main problems is folks don't know what the heck they are doing - they buy cone/mist sprays and try to use them in outdoor conditions where the mist is easily dispersed or, blows back on them; they deploy with a cone/mist spray and then move into the area they just sprayed, sucking in the particles. Or someone else does it for them.
For self-defense OC sprays I would recommend a streamer versus a cone/mist pattern. The stream of agent gives you more range, is more likely to put OC on target because you actually see the stream and can aim it. You want to hit the eyes and the mouth - preferably with the eyes open and when the person is inhaling - so it needs to be a surprise. For the best incapacitation possibility you need to deliver 2 to 3 seconds of OC on target and then disengage.
OC works on pain compliance and to a lesser degree on the inflammation of the respiratory tract and the membranes around the eyes. I will tell you that MOST of the failures reported with OC are due to insufficient quantities delivered to the target areas.
Don't be fooled though, a determined/drugged/angry person can fight through OC, especially if they have experienced it before - if I had a choice between OC and TASER C2 for personal defense, there would be no choice, I'd take the C2.
TASER - the civilian TASER's are the X26C, the M26C and the C2 - the C2 is the one purpose designed as a civilian self-defense device, it is also roughly 1/3 the price of the other models. It gives you a standoff window of point blank range to 12 feet (15 feet is maximum range - limited by wire length) and then gives you a 30 second window to escape. If you file a police report TASER will replace the system, all you pay is shipping.
Nothing in life is absolute, the TASER's biggest detriment is that for incapacitation to occur both darts have to lodge either in the person's skin, or in the person's clothing with no more than a one inch gap between clothing and skin.
In terms of something guaranteed to stop an assailant at close range I'd place the TASER above the handgun, unless you are able to deliver CNS shots reliably under stress.
http://www.taser.com/products/self-defense-products/taser-c2?gclid=CP7RxrvR3bECFSpgTAodDCEA2A
Some folks have mentioned stun guns, they are different than the TASER, they use high voltage, low amperage electrical charge to case pain, they don't incapacitate as the TASER does, plus you have to be in close physical proximity to the attacker.
Batons were also mentioned - as I mentioned, I've been a police trainer for over 30 years, most officers don't stop someone with one baton strike, do you really think the civilian average would be better?
In terms of the self-defense the original poster was talking about, I'd go TASER C2, knife.
I'd also get "The Gift of Fear" by Gavin De Becker and make it required reading for everyone in the family.
Hope this helps.