1st off don't ever ever ever let anyone jack with your computer.
You have just learned a valuable lesson of people jacking with your computer and they don't know what they are doing.
You can update your Mac to a newer operating system, the newest is Snow Leopard OSX 10.6
No doubt you have an earlier version. When doing this you will loose everything you have in the way of programs you have now.
You will have to purchase any upgrade to a newer operating system.
It sounds like you may have either OSX 10.3 or 10.4
The easiest fix is to get out the disc that came with your Mac and re Download the operating system you already Put in the CD and press the C key and follow the instructions on the screen. Do Not choose Archive Install if you have programs that he has deleted portions of. Choose Erase and Install.
After you do it go to the Blue Apple upper left click it and choose Software Update and download all the updates for your computer.
Your other option is to purchase a newer version of OSX. But, before you do go to apple's web site and check and make sure that your version of computer meets the bare system requirmrequirementsll even install on your system.