They gave bad advice that probably made you spend money you didn’t need to. Since it’s 6 years old it may have come with a OS DVD.
1. Start the Mac and insert the DVD.
2. cut the power and start it hold the C key or option key and have it boot from the DVD.
3. Run disk tools and repair the hard drive and permissions.
4. if that doesn’t fix it you will probably have to re-install the OS
If you don’t have System DVD try booting in safe mode (look it up) sometimes doing that alone will fix the problem and the next boot will be normal.
We don’t have an OS DVD :0(