Operating System Backup

I'm sure this question must have been asked many times before so I'm sorry but could not find anything by doing a 'search'.
I have two hard drives on my PC with Windows XP SP3 installed on primary drive 'C'. I have the licence number but never have had the installation disc. As the PC is getting on a bit I'm worried that sooner or later a hard drive is going to go belly-up and I will be left with a situation where I can't recover my operating system.
This is probably a naive question but what is the best thing for me to do to safeguard against this. Can I somehow load the operating system onto the second drive and have it sitting there as insurance or is it possible to make, beg, borrow, steal or (last resort) buy another installation disc ?.
What do I need to do to help me sleep easy at night !!, Thanks