It's worth maybe searching acer's support site. I'm afraid i'm not that familiar with Acer PCs/Laptops so i'm not sure why you can't change it. But it's obvious they have got some program running that's stopping you changing it.
If you still have no luck trying safe mode, download a program called
Hijackthis and run it with the report option. Once it's run it'll open notepad and you can copy the entire document and paste it into the box on the
http://www.hijackthis.de website, which will look at the file and check your settings.
Good luck, let us know any outcome