idle dropping
 

idle dropping every time i come to stop and , my check engine light is on and the code is vaccum leak but can not find leak. and when my fan come on idle will drop from 800 to 200 or less , i dont know what is causing it . is this my idle air control valve

No, the symptoms for a bad idle air control valve include a bouncing idle. If your intake manifold gasket is leaking, you may be able to tell by shooting some fuel injector cleaner where the intake interfaces with the block (while the engine is running). If the stuff gets sucked in while the engine speed changes, your gasket needs replacing.
Of course, some vacuum leaks in tubing are hard to find, especially when the tubing is hard from age. I recently replaced a whole bunch of vacuum tubes in my 89 Mazda. The biggest leak, though, was from the intake manifold gasket.

If a 1995 model, it's a MAP controlled engine-- vacuum leaks will cause the idle speed to increase. The IACV is probably clogged or sticky. Always start by cleaning it.

i have a 99 civic vtec ex

it dont bounce it just goes down after coming to a stop that is the only time i think the idle goes down down when my fan comes on . then when i use my automatic windows it takes power from the car , idle goes down

Then I would follow Mk378's suggestion.