I scanned it, and right now, ALL of the files could not be scanned. Is this normal? It is my first time to use it. I have Avast 5, and is this cleaner suitable for it? Thanks.
No… the Cleaner is an obsolete tool… it can detect and clean a minimum amount of infections (although it does well what it’s supposed to do: clean). Stay and scan with avast 5 only 
avast virus cleaner have a limited detection capability, if i dont remeber wrong it is stated on the gui. and the program have not been updatet for a very long time. you dont need it if you have avast antivirus installed
Unable to scan
http://support.avast.com/index.php?_m=knowledgebase&_a=viewarticle&kbarticleid=47&group=eng
If you want a Second Opinion, try Hitman pro
Hitman Pro 3 - Second Opinion Malware Scanner http://www.surfright.nl/en/hitmanpro