Can't hide .app extensions

New convert from WIndows, enjoying the experience very much so far. Am getting to grips with my iMac and Lion but there is a small annoyance that has appeared that I'm sure wasn't there a couple of weeks ago - in Finder all my applications are shown with a .app extension. I went to Preferences and unticked the relevant box, at all is OK until I log in again and, you guessed it, they're back again. Any suggestions? No big deal, but it niggles.
Thanks in anticipation.

Run Disk utility to repair permissions.

After that, restart the Mac and hold the shift key down. Keep holding.

You'll eventually get to the login screen. You'll see the words "safe mode." You can release the shift key and log in.

Once logged in, do nothing but restart the Mac (this procedure forces an internal systems reseting -- don't worry, no data is at risk).

That should clear up the issue.

Many thanks chas_m, I started the process but can't seem to startup in safe mode. On restart I hold down shift after the startup tone, then release when the logo and process bar appear. Other than a slower startup and a slight initial screen flicker there is no indication it is in safe mode. I tried this routine a few times but no different. I still have those pesky .apps showing. Any thoughts?