For prevention, there are a few things you can try. Paint the basement wall and floor with a sealant. Then most important of all, make sure you have positive drainage away from your exterior walls. Probably a landscaper can landscape your grounds such that the rain water always flows away from the house exterior walls. In addition, make sure gutters are cleaned maybe three or four times a year. Also go to a hardware store and buy an extension for the downpspout so that the out flow is further away from the wall. These few things will probably run you about $500 or so. (I did all except the sealant for my basement leak. My former neighbor did the sealant thing for his basement because he noticed a crack in his unfinished basement).
If you call a contractor, they will probably suggest putting in a sump pump and interior drain (probably cost $5000 or so and will not be as effective as the above suggestions).
Good luck.
