Your warranty is only void if you open the notebook up. What software you choose to install is your business, not Acer's.
Anyway, yes, you can reformat it with the true recovery and it should be back to normal. Windows sometimes isn't very happy with partition resizes.
Else, you can always reinstall Windows with an OEM Windows disk, using the key on the bottom of your notebook to activate. That should allow you to configure the partitions however you choose. You will have to install all of the Acer software (if you want it) and your drivers manually, however.