does anyone know where the key or how to get into the locked basemant in a ruined house in springvale,the second house on the right as you first come into springvale after leaving vault 101.

It's part of a side quest after you have completed the Main Quest line. Apparently the quest cannot be activated if you blow up megaton.

If you want to look the quest up it's called holy water

It's for a side quest that was added by Broken Steel so first you have to beat the main quest and then after the 2 week long nap you take you can either speech challange a BOS scribe at the water purifer or go to underworld and talk to the ghoul selling some type of special water to start the quest.