What avast version are you using 4.8 or 5.0 ?
What is the malware name, the infected file name, where was it found e.g. (C:\windows\system32\infected-file-name.xxx) ?
You could try booting into safe mode (as the boot-time scan isn't currently available in 64bit OS versions) and run an avast scan from there (use the desktop icon to start avast as it doesn't run in safe mode by default..