1979 Monte Carlo won't start...

I've got a 79 Monte that won't start...after having let it sit for about a week, it went from the engine revving, to a quiet grinding/drilling noise when I turned the key, until now nothing happens at all. What are some of the reasons this could be happening? I checked the battery voltage and I see it drops from 10v to 3v when I try to start it, and another clue is the electric windows won't work now. Could it be the battery? I'm planning on taking it to a mechanic soon, but I want a heads up on maybe why...thanks.

(By the way the carburetor needs to be adjusted, and a cover and air filter)

Very tough to tell what this noise could be over the net, no? Best guess, starter drive is bad or the flywheel.

Update- it now won't start at all, I checked the battery and it drops from 10v to 3v when I try to start it, and the power windows won't work.

Have the battery load tested, sounds weak. This is all basic diagnostics.

First you need to fully charge your battery. Charge using the 6 amp rate, charge will take 2-4 hours, so be patient. Check voltage, you should have 12 to 12.4 volts. If you do not have at least this much voltage, your battery is bad. If it has at least 12 volts, try starting again. Trying to crank with low voltage can damage the starting system too.