If you are going to ask for more information, it would help the OP to say what you want.
What is the infected file name, where was it found e.g. (C:\windows\system32\infected-file-name.xxx) ?
For detection on on demand scans, check C:\Documents And Settings\All Users\Application Data\Alwil Software\Avast5\Log (Windows 2000, Windows XP). Or C:\ProgramData\Alwil Software\Avast5\log (windows Vista, windows 7).
- You could enable a boot time scan. From the avastUI, Scan Computer, Boot-time Scan, Schedule Now button and reboot.
Look in the C:\Documents and Settings\All Users\Application Data\Alwil Software\Avast5\report\aswBoot.txt file (XP location) C:\ProgramData\Alwil Software\Avast5\report\aswBoot.txt (Vista, Win7 location), check this file using notepad for info on the scan/detections, etc.