First off, I find it hard to believe that a member with over 800 rep points would be a asking a question as simple as 'My case fan is dusty, what can I do?'
It's a case fan, use compressed air and move on. If it's still overly dirty afterwards just remove it and run the blades under the faucet for a couple sec, then finish it off with a wet wipe or something. And No, water will not hurt that fan. The motor and wiring are well protected. Just make sure the blades are 100% dry before you plug it in and have it fling water all over your other components (the ones that water will hurt). Don't worry about greasing the ball bearings in those things as you're more likely to damage the damper pulling it apart than performing anything useful.
If after all this you find that the fan is still spinning slow/stopping, then it's a power distribution issue. Either you have too many things plugged in tapping your PSU, or a loose connection in the molex daisy-chain (if you're set up that way).
In the end- it's a 5 dollar fan.