Without knowing the model, I can't say what would and wouldn't work... But typically, only the motherboard case and maybe PSU would be the only things you couldn't "reuse", and that's becoming quite rare anymore... However, also depending on it's age, you may not be able to use the RAM(say if it's DDR and you need DDR2) hard drive(say it's IDE and your IDE channel is taken by optical drives) Or such...
what are you planning to use this for? low gaming? med gaming? word processing? photo/video editing? extreme gaming?