Please help me…
I have a NEC laptop, Pentium IV and a 512 MB of RAM with an operating system of Windows XP SP3. I was using avast for a long time and it perfectly solves my problems. But until such time I’ve found out that the Standard Shield of Avast won’t scan any files. The blocker also does not work. I’ve tried reinstalling Avast but it won’t solve the problem. Please help me, the problem is causing much risk to me. 
How did you find this?
Is there a red cross in your ‘a’ blue icon in system tray?
Is ashServ.exe running? (ctrl+alt+del to see the processes running).
Did you have any other antivirus in the past in this computer?
What avast processes are running in Task Manager, they begin with ash or asw, see image ?
The Blocker is disabled by default, so I wonder what other tweaks you have been making in avast without knowing there effect ?
However, answering Tech’s questions my throw some more light on the problem.