Well, if you weren't confident enough to follow steps given to clean your HD, then you did not get the cleanest possible reinstall this time. And you have the windows.old folder because you didn't even format the partition you reinstalled Win7 to. After making sure you have all of the files you need from it, you can delete it using Disk Cleanup.
So how is the performance of your reinstall? I suggest you install Microsoft Security Essentials for the most effective lightweight free AV, check Optional Windows Updates for newer drivers, then check your Device Manager for missing drivers which you can find on the Acer Support Downloads webpage for your model.
Install programs slowly over time so you can check performance after each. Don't let any programs freeload on your Startup which is the biggest cause of peformance lag - I uncheck all but AV and gadgets in msconfig>startup tab (type msconfig in Start Search box).
When it is set up as you wish, run the CCleaner "Run Cleaner" and Registry tab, then the Auslogics Disk and Registry defraggers to clean and order your HD perfectly. Then save a Win7 backup image.
If you find performance lags any after all of this then consider wiping the HD as suggested to get the cleanest possible reinstall. I only suggested this as I know first-hand it works best with Acers since I have two at home.
