Welcome to Fixya Support
I am Michelle, and i will be assisting you regarding the issue
The issue you have mentioned indicates a problem with the operating system or the hard drive itself.
The system is not able to load the operating system either due to some corruption in operating system files or due to some fault with the hard disk itself.
To diagnose and fix the problem i will list down a set of steps below. Please try them
1. please restart the system and tap the F8 key on initial system logo.
in the advanced menu select the safe mode option.
login as an administrator
click start...in the search box type system restore
in the window that appears, restore computer to earlier date and time and click next
select a date may be a week or 2 back when system was running fine and click next
follow the onscreen steps
2. In case you are unable to perform the above step or in case it does not work, please perform a repair of windows vista
please click on the below link that has steps with pictures for easier understanding of the process of vista repair
Once done your system should be able to boot backup normally
3. In case not it is an indication of either a corruption with operating system which means that you need to load a fresh copy of windows
below link has the steps mentioned to do the same
(Please note that when you load a fresh copy everything on your system gets wiped off and then you need to reload everything back on)
4. In case all of above fails then its a hard disc fault that would require a physical repir\replacement by a repair technician. If the system is within warranty please contact Acer Tech support for hard drive replacement else you may take it to a local computer repair store.
Please feel free to ask in case you have any queries and i will surely try to assist you in the best possible manner.