All Macs at that time came with install disks and the previous owner should have passed them on to you.
You can call Apple and quote the derial number of your Mac and ask for replacement disks, which they can send you for a small fee, or you can buy a retail Tiger disk on eBay or Amazon, but that will cost more.
Restore Tiger 10.4 & Leopard 10.5 DVDs are available from Apple by calling 800-767-2775 as of January 20, 2013.
What programs are you talking about?
Some iMacs were shipped with productivity apps -- iLife, iDvd that may not be on the above installation discs.