I started my washing machine this morning, walked away, came back a few minutes later and my entire laundry room had turned into a shallow pool. Water was still pouring into the machine, I turned the washer off but the water didn't stop. So I had to turn the water off at the hookup on the wall. It's a GE and is pretty old (7 years?)and had been "rebuilt". I bought it used a few months ago. It worked fine, the agitator was quite loud but I got use to it, which is another problem. Why isn't the water shutting off when its full and why is the agitator so ridiculously loud?? Is it easily fixed?

I would cut my loses and start looking for another machine. The water flow is controlled by a timer motor and a solenoid to close the water valve. One or possibly both have failed. The noisy agitator is likely a bad transmission and throwing more money at this old guy may be a bad bet. Did they warranty the rebuild?

If water overflowed it is more than likely the pressure switch or water valve. You need to check bothe. Each can be purchase for around $35 each. My guess would be the pressure switch. Like ball suggested,putting money in an older used unit is not the best thing to do. YOu can buy basic new ones at the big box stores for around $350

why isnt the water shutting off when its full and why is the agitator so ridiculously loud????? is it easily fixed?
If what you stated is correct ("i turned the washer off but the water didn't stop.") the electric water valve is leaking.
This most of the time is related to dirt in the water blocking the electric valve to close.
To sort out the problem out :
1-empty the drum (keep water tap closed, use water pump of washing machine)
2-open tap for 1 second, close tap for 1 second, repeat that a couple of times.
3-if that sorts out the problem, know that your water supply contains sand and stone, and that this can happen again. You can buy an in-line filter to prevent dirt from entering your washing machine.
4-Do not forget to clean and/or replace the electric valve soon.
5-Always disconnect the power lead before you open a washing machine. Safety first.
Note :
If the first line of this reply was not correct, than the problem is located in the pressure switch, a flat roundish can with a rubber hose connected to the drum, that controls the water level in the drum.
Contact me if you need more information. For info on the agitator I need more information to advice you.
Lets first solve the water problem!
Success!
;)