Everything Dies and case fans are no exception. You can help them last a bit longer by periodically cleaning and Oiling them. For this I am using some Sewing Machine oil, but light machine oil of other types should be fine.
Most case fans that a sticker on the back that can be pealed off to reveal the lubrication point. This fan has no cover on the lubrication point
This fan has the sticker as well as a rubber cover on the lubrication point
The small rubber seal can be easily removed by prying it out
Add one or two drops of oil, spin the fan a few times, add only one or two drops more
Soak up any excess oil so that nothing spills out of the fan
Clean the oil off the surface where the sticker goes, consider using some rubbing alcohol to ensure that all oil is off of it
If you have a fan with a plug, then put the plug back in, if not skip this step
Put that sticker back on
Plug them back into the mother board and revel at how much your computer blows