Vista stuck booting on the green bar

Awhile back i had to do a hard reset due to a freeze. When the computer resetted it hit a blue screen and automatically restarted. I did this about twice untill I tried the startup repair i clicked the top option and it seemed to fix itself due to the system being able to properly restart again. Recently my computer has been becoming a little laggy so I tried what my friend told me and got the System Nortonworks trial and scanned my computer it seemed to fix everything the lag seemed to have lessened. Now yesterday night my computer re-hit the blue screen after a freeze hard reboot the problem seemed fix at the time and the system began to work a little faster so I thought everything was okay. I left the computer on overnight just in case and slept. This morning i tried to get into the start screen but it didnt work. I was testing out various buttons untill i got this error

MSASCui.exe

The instruction at 0x7449e84e refferenced memory at 0x7449e84e.The required dat was not placed into memory because of an I/O error status of 0xc000000e.

Click ok to terminate program

So i clicked ok and still start and no other programs worked.CTRL ALT DEL didnt work eather so I did another hard reboot. After that my screen has been stuck at the black sceern with the green bar where the C Microsoft corperation screen. When I try to do another hard shut down at this point it gives me two option to try the startup repair or to start the desktop normally. They both would get stuck at the green bar.

I am using a Dell Inspiron Desktop 531.

Any help would be appreciated.