If you look at the avastUI, Real-Time Shields, Web Shield and the Shield info, Last Page Infected: field should give that if you haven't rebooted since then ?
Use notepad to check the C:\Documents and Settings\All Users\Application Data\AVAST Software\Avast\report\WebShield.txt (XP) or C:\ProgramData\AVAST Software\Avast\report\WebShield.txt (Vista, Win7) file. This may well be a hidden area so you would need to change the windows explorer, tools, folder options, Show hidden files and folders setting.