Is the DVD you reinstalled with an Acer-branded Windows 7 Reinstallation DVD? What does it say on it exactly?
An Acer reinstallation DVD likely has SLP activation baked in which avoids the need to activate in the same way the Factory Recovery Disks (loaded with bloatware) do. But using one meant for another computer could foul activation.
You might be able to change the Product key to the one on the COA sticker at Computer>Properties. If not, open an Elevated Command Prompt,
type slmgr -ipk
followed by a space and the COA Product Key. Then activate at Computer>Properties link or running another command to slmgr -ato
If this fails and continues non-genuine prompting then you'll need to start over with a retail Windows 7 DVD following the steps I gave you to get a Perfect Reinstall.
To further answer your question, retail and OEM installers are the same starting with Vista. The issue here is possible SLP factory activation meant for another machine interfering with COA activation.