Q: How to reinstall again Yosemite, my Mac is back to Mac OS clasic?

Hello guys My MacBook Pro Hardware Overview: Model Name: MacBook Pro Model Identifier: MacBookPro5,3 Processor Name: Intel Core 2 Duo Processor Speed: 2.66 GHz Number Of Processors: 1 Total Number Of Cores: 2 L2 Cache: 3 MB Memory: 4 GB Bus Speed: 1.07 GHz Boot ROM Version: MBP53.00AC.B03 SMC Version (system): 1.48f2 Serial Number (system): W8*******642 Hardware UUID: ***************Sudden Motion Sensor: State: Enabled I download the last yosemite update and after instalation Mac was just dead, he was asking me for reinstall yosemite but he was showing the error non stop and the time zone was totaly different, was not able to make it right. Because of my work i need to reistall OS and put my DVD which is come with my Mac, yeah but he reinstall and put my back couple of years :} I wana ask how to install back Yosemite now, did I need to buy again Mountain Lion or some other OS, or just you have a solutions. Thank you