Usually Alt + F10 key combo at boot works with the newer acers to factory restore, but since you installed Windows 7 on top of the oem windows, the acer custom mbr would have been replaced by the standard windows mbr and that method wont work. From your post it doesnt appear you've made the recovery disk set either (though if you have, you can use those to factory restore). Easiest option for you now would be to order a set of recovery disks from acer for a small price, then use those to factory restore. before doing that back up any important data stored on the hard disk.
FYI, this is not the end of the world. OEM installs are just the base OS with some propreitary apps and trialware thrown in along with the device drivers
of course. The drivers and useful apps can always be downloaded from the support page for your model on the acer website, the trialware expires anway so no point grieving over it. If you're upto it, now that you've installed windows anyway, you can make use of it. Just download all thats required and install. To activate (assuming you've installed the same edition that came preinstalled), use the sticker key with phone activation. Activate Windows 7 by Phone
Once you're activated and nicely set up, you can save an image of the install for future uses (in case of other such, errr, accidents).
BTW, what kind of install disk did you use? If you actually used the acer recovery set, you're fine and you can ignore the above.