You may want to hit Acer with an email to confirm but I'm pretty sure the Windows 8 product key is embedded in the BIOS (firmware) of the computer.
So if you have the installation media or created the system recovery DVDs as suggested when you first brought it home you should be good to go.
If not then you may need to ask Acer for their recovery media for your specific laptop.
EDIT: Additionally, depending on how you went about installing Windows 7, you may still have the factory recovery partition on your system's hard drive, in which case you may be able to use that for a factory reset.
Of course, backup any critical data before you go forward with the process.
