Backup your good .cfg file somewhere safe!
You can delete or overwrite the .cfg. It's stored under
C:\Documents and Settings\username\Application Data\Microsoft\FSX
FSX should create a default one when you next load.
The ifly 737 is simply awesome, although I can't comapre it to the PMDG one as I haven't used it.
If this is going to be the first complex plane you have, please be patient when learning it. I had a really good stepping stone from the Flight 1 ATR.
Here's a quick runthrough of what I personally found with the product.
1. The FMC takes some getting used to. It helps if you can load in a pre-prepared flight plan. I use
http://www.simsrd.myzen.co.uk/ for a UK one. I then paste the route into AivlaSoft's EFB. Save it as an iFly Plan. Then load this from the fsx ifly menu. The plane does come with some pre-prepared flight plans.
2. A deliberate thing is that the parking brake needs to be on before you can select 'ground power' from the fsx ifly menu.
3. Learn how to start the APU.
4. Learn how to start the engines.
5. Learn how to use the Boeing MCP.. E.g you can dial in 18000 ft in the 'alt' window, but nothing is going to happen until you tell it how to climb or descend to that alt.
6. Bleeds off takeoff - worth knowing, but not necesserily needed for the NG
http://forum.avsim.net/topic/368101-engine-bleed-off-takeoffs-policy/
7. ANY movent on the joystick is going to disable the AP. Unless you tell it not to in the ifly Config Util.
8. Learn about LNAV and VNAV.
9. Autoland is included
10. Learn and master trimming. AP won't engage if your plane isn't trimmed. Unless you tell it not to in the ifly Config Util.
I was flying the 737-800 in Ryanair colours, then it was only recently I realised that the 737-600 is included and have been loving that at shorter fields. So I've now got the Pan-Am livery.
http://library.flight1.net/?cat=3
This may help:- ifly Checklist
http://library.flight1.net/?p=622
They keep on rumouring that there's going to be an update at somepoint. We're still waiting.
Ask away if you have more questions.