I have a odd - stale - odor coming from my vents when we turn on our airconditioner. I have changed the filters and it doesn't seem to help. Any suggestions? Could it be coming from condenser outside?

Is it a small window unit?

If so I run hot water through it before and after each summer to clean it out, you might try that, just don't plug it back in for a good 2 days after this.

It's most likely just from the water laying in the coil pan all winter. Most of the time after you run the A/C for a short time it will go away, if it doesn't you may want to have the coil cleaned. It may also be ( if your return ducts are in a crawl space) a return has came loose and drawing musty air from the crawl space. A little more info on the system might help?