I very recently installed AVG 8 with no trouble.
I think you'll find that the scanning of individual incoming files including email, is automatic.
I've just checked this with an old backup CD which has a copy of Happy.exe "Threat Detected!" was the message with details of the suspect file.
Although this program is well behaved and dosen't replicate or take over, it must have some viral like code and it stays under wraps!
Default settings are for regular scans which on a well ordered PC are not necessary..... well if done manually when you can leave the thing for a while!
I simply disabled regular scanning because I believe my system is clean and a pop-ups happen if there is any threat.