Backing-up is NOT a choice. It is mandatory.
You need an external hard drive. At the very least, get one that is the same size as your computer’s hard drive. An 80GB drive costs less than RM300.
Then, back-up regularly. Set a fixed time to do this every week or perhaps daily (overnight). Use the Time Machine feature, if you are on Leopard. Time Capsule’s available soon and you may want to have a look at that.
Ignore this advice at your peril.