Am I eligible for interest off my security deposit in Alexandria VA? How would I know if so How do I calculate this amount from July 2006 till October 2007?

You don't get interest OFF, you get interest added TO your security deposit, but only if you have been in the residence for more than 24 months. You can read more in the VRLTA:

http://www.arlingtonva.us/Department.../pdf/vrlta.pdf