If you have not already, you should create a set of Acer factory recovery discs to have as a backup to be able to do a factory restore of Windows 7 with if you like. The link below is for Vista, but I believe it's the same method for Windows 7.
How to create ACER Vista Recovery disks
Next, it sounds like you HDD needs to have the clean
command run on it, and then reinstall Windows 7. You can use the tutorial below for how to do a clean install on your computer.
Clean Reinstall - Factory OEM Windows 7
Before doing anything, make sure that you have your product key number
on the COA sticker on your computer available, backup anything that you do not want to lose to a separate HDD, and have your drivers' installation files downloaded from Acer for you specific computer model number and saved to a separate location in case you may need them after installing Windows 7.