Reinstall OS on GT-S5830i Is it possible to re-install the OS (Android 2.3.6) on to my GT-S5830i? 

If yes, how can I get that done?
Yes.you can reinstall the OS.

There's a low level flash tool called heimdall (or Odin) that can be used to reinstall the firmware or install aftermarket firmware onto the Galaxy Ace.