I used regular household bleach at rate of 6 fluid ounces per 10 gallons of water. Be careful to read the bottle. There is a new product that has bleach and cleaner in it that is not at all suitable for disinfection. It is also important to note that I use the tank water for washing only. I use only bottled water for drinking and cooking.
Bleach (dilluted) is the way to go. Run it through the system, until you smell it coming from the faucets. Let sit for at least 4 hours, but not longer than overnight. Chlorine is very corrosive and will eat away your system, if you leave it too long. Then rinse the system with more than one full tank of clean water.
Old systems will typically have some green mold that builds within. This method may kill the organisms, but it won't remove them. Getting inside your tank and cleaning by hand is a good idea occasionally. The only real way to clean hoses that have mold buildup is to replace them.
More is not better. The advise of 6 oz. of household bleach per 10 gallons will give a solution of ~ 200 ppm, which is about optimum for a dirty system. Higher levels will increase the pH excessivly and actually decrease sanitizing effectivness.
This is also about 100 times more than you will want to use for routine use. The perfect level depends on local water conditions, but will be the lowest level where there is any detectable chlorine residual.