most ide hardrives last about 6 years, if they're running constantly, you can set the power management to turn off your hds after you being away from your comp for 10mins or more. Doing this will go along way to keeping your hd healthy.
If your hd just started to grind, and you decide to do nothing. Chances are the hd could last another 6 months or so, but you will notice it to start to slow down. Try going on ebay and finding a hd of around the same size and model. This way your reduce the chances of having to flash your mainboard because it won't except the bigger and newer model hd.
Ever have an old oscillating fan that started to slow down and make noises? Did your folks throw it out or just take some machine oil and lube it up? People do fix hds that are going it but it requires a clean room and specialized machinery. You can get your hd fixed probably for a 300-1200 bucks depending on how you want your information back. On cd or in a completely rebuilt hd.