This is vary easy way.. Today I do this...

Things You'll Need:
Acer PC or laptop
Step 1:
Click "Start" menu.
Step 2:
Click "Programs."
Step 3:
Click "Acer Recovery Management."
Step 4:
Click the "Restore" button on the bottom of the dialog box that opens.
Step 5:
Click "Restore System to Factory Default."
Step 6:
Click "Yes" in the warning dialog box which asks if you wish to continue the recovery process.
Step 7:
Click "Start" to restore the computer to the default factory settings.
Tips & Warnings
If you are unable to access the Start menu, use the recovery CDs that came with the computer or visit Acer's website (see Resources below).
Always make a back-up CD before restoring a computer to factory defaults.
Acer Website

Source: i have an acer aspire