A. No need to worry unless an all out nuclear war occurs, with attendant fallout.
B. In that event there is no need to worry if you have a reasonably deep well (more to worry about from chemical/fertilizer contamination in this case).
C. A reasonably effective particulate filter can be made from commercially available filters & housings, including a carbon stage that will filter particulates in the 1/2 to 2 micron range. This would likely be effective against even some viruses and could likely be used for biologically contaminated surface water (with a pump to push the water through the filter).
D. There are many levels of water treatment ranging from none through water softeners to filters to reverse osmosis, all have some advantages in some situations.
E. If you are truly paranoid, you can deionized and triple distill your water to remove all but the chemically pure H2O (although this runs the risk of removing necessary minerals from your diet). If you actually seriously consider this then I recommend that you see a professional therapist.
For the only moderately paranoid just install a 2 stage particulate/carbon filter in your waterline and service it regularly. A handy person could probably do it for less than $100, although you should check your water first to see if it actually has a problem that filtration will fix.
If you are worried about your children/grandchildren because of the reactor problems in Japan, don't; they will probably not get as much radiation as we were exposed to in the 50s during the Nevada nuclear testing.
The best thing to do when worried is to educate yourself on the actual danger and possible consequences of any problem. Use information from reliable, non agenda based sites not places like here where many have apparently stopped taking their medication and/or are promoting panic because they are ignorant, poorly raised children (that think its cool to scare people).