Hi yes you can install snow leopard on you machine up to 10.7.5 I think.
You OS (operating system sound corrupted) maybe a bad down load !.
Do you have the original OS disc for your computer ?.
If yes, with your machine switch off, put the disc into your dvd slot, hold down the C KEY and start the computer up, when the installer card shows, at the top LEFT HAND you'll see the tool bar, under Utilities look for Disk Utility from there you find First aid, Erase, Partition, Raid, High light you hard disk showing on the left hand panel then under Erase you'll find Format drop down which you set to Mac OS Extended (Journaled) name your Hard disk press Erase, (your hard disk is now formatted for a clean install, close the Utilities down (red button top left) you can now carry on and install your OS.
If you have a large hard disk and you want to Partition it, do it from hear to, press the Partition button select the number of parts you want, make sure format is set on Mac OS Extended Journaled press apply close Utilities down install you OS You'll need the first part about 8 GIG for your OS.
best of luck mate.