I am operating MAC OS 10.3 version processor 800 MHz PowerPC G4
Memory 512 MB SDRAM

I have the instllation CD but I dont know if I should use it to reinstall the entire system or simply upgrade what I have
It is slow, how do I check it to see if the hard drive needs to be replaced?
how do I check it for spyware, malware, virus etc?
Why cant I get it to receive chat messages? is this something I need to install or what
Please only answer if you have good sound advice, something like buy my daughter a new computer really is not answering my questions and certainly isnt any help
thank you