Do you have the installer discs that came with the computer originally? If so, then boot from Disc One, erase the drive, then reinstall Snow Leopard. You can then download Yosemite from the App Store using your Apple ID. Note that you no longer can download Mavericks.
If you don't have the original discs then it's possible to do a Network Recovery if the computer is qualified: Computers that can be upgraded to use OS X Internet Recovery. You now can do the following:
Install OS X Using Internet Recovery
Boot to the Internet Recovery HD:
Restart the computer and after the chime press and hold down the COMMAND-OPTION- R keys until a globe appears on the screen. Wait patiently - 15-20 minutes - until the Recovery main menu appears.
Partition and Format the hard drive:
- Select Disk Utility from the main menu and click on the Continue button.
- After DU loads select your newly installed hard drive (this is the entry with the mfgr.'s ID and size) from the left side list. Click on the Partition tab in the DU main window.
- Under the Volume Scheme heading set the number of partitions from the drop down menu to one. Click on the Options button, set the partition scheme to GUID then click on the OK button. Set the format type to Mac OS Extended (Journaled.) Click on the Partition button and wait until the process has completed. Quit DU and return to the main menu.
Reinstall OS X: Select Reinstall OS X and click on the Install button. Be sure to select the correct drive to use if you have more than one.
Note: You will need an active Internet connection. I suggest using Ethernet if possible because it is three times faster than wireless.
