Defrag basically put the files together in one place http://en.wikipedia.org/wiki/Fragmentation_(computer)
Your friend google http://lmgtfy.com/?q=what+is+defragg … 
does it gets the file back in c drive if any important files deleted by avast while removing virus.
Avast should not delete any system critical files
wat happened to my sound system?
it not working from when i run those softwares which u said.
i tried to check my mp3 system and woofer connected with my pc. they are working but from pc sound system not working.
also i cant hear mp3 songs.
wat change i have to make now?