You may be able to upgrade your operating system. It depends upon the hardware. Get more information about your computer. Go to the Apple in the upper left corner of any window, then "About This Mac". Write down what it says about "version"and report that here. Now continue to "More Info..." Copy and paste the information here, but omit the serial number and Hardware UUID (if present).
Ultimately realize you are running an old operating system. Apple gave up on supporting Tiger about 3 years ago (that's about half a century in human years). Things will only get worse. Every 5-10 years I am forced to buy a newer used computer not because my old one no longer works but usually because they change browser requirements for me to read my e-mail. It's a single thing, but something essential. Hotmail also just changed things to be very unfriendly to old Firefox users.
Mac OS X 10.4.11
Processor 2GHz Intel Car 2 Duo
Memory 1GB 667 MHz DDR2 SD RAM
Model Name: iMac Model Identifier: iMac5,1 Processor Name: Intel Core 2 Duo Processor Speed: 2 GHz Number Of Processors: 1 Total Number Of Cores: 2 L2 Cache (per processor): 4 MB Memory: 1 GB Bus Speed: 667 MHz Boot ROM Version: IM51.0090.B09 SMC Version: 1.8f2
With your computer you can upgrade to at least Snow Leopard (a good start) which will allow you to run much newer software. Start with Snow Leopard. The main other consideration is loss of use of some older software if you have a big collection of old applications.
Snow Leopard 10.6 Technical Requirements - http://support.apple.com/kb/SP575
Macs and Software that will run with Snow Leopard (Mac OS X 10.6.x) - https://discussions.apple.com/docs/DOC-2455
A Mac OS X 10.6 Snow Leopard Application Compatibility List - http://snowleopard.wikidot.com/
Kappy 08/2012 post on upgrading to Snow Leopard, then Lion or Mountain Lion - https://discussions.apple.com/message/19401628 - including how to get Snow Leopard and Lion since Apple removed them from the online store.
I would seriously consider getting more RAM ("memory") for your computer to run a newer operating system version. You can get more advice on that by posting in the iMac (Intel) forum with the computer information you posted above.
The tips Michael Wasley provides are good as temporary measures. The second one especially is just a trick that may or may not work. You will find it a lot easier to just run newer software on your computer. Snow Leopard is relatively inexpensive ($30 from Apple, more from other places). The memory upgrade will cost more but you can get more information on the other forum.
The single most useful accessory you can have for your computer is a good backup hard drive. With a good backup you can install new sofware and not have to worry if something goes wrong because you can always return the computer to the pre-install configuration.
To start with, you need an external drive. There are many options available and I am not 100% up on which are the most reliable models. And external drive is really two parts. One is the enclosure (box with electronic chips). The other is the actual hard drive inside (which looks like the one inside your computer). While Western Digital (WD) make the most reliable hard drives (currently) their enclosures are not the best. You can look at something like http://eshop.macsales.com/item/OWC/MAU4S7S1TB32/ OWC brand is pretty reliable as far as the enclosures go. However, I note with this link I posted that the drive inside is a Hitachi drive which is not getting all that great a review on the web. if you were interested in that OWC drive I would ask them about getting an upgrade to a Western Digital blue or black (black > blue > green in terms of quality but also the order in terms of price).
Yes, this is money but it is probably as important as your computer itself. If you have digital photos on your computer you no longer can make prints from film. If your computer dies or is stolen tomorrow it is all gone, 100%.
You can look for cheaper alternatives. Seagate used to make some of the best drives but was bought out and now makes some of the ones with the worst reliability reviews.
Ask your son but if he uses a Microsoft system PC and not a Mac then understand things work differently and there may be misconceptions on what is best.