i think several problems…
- you were infected by a “System Volume Information” Virus, so the numbers were wrong(?)
- [s]there was still unallocated data space left/s
- any other drives attached? usb drive maybe? external harddrive?
- what’s the current capacity of you harddrive?
why did you do a boot-scan?
what OS are you using?
what version of avast are you using?