1) These malware files are changed every few hours so nothing can detect them- I've sent samples to VirusTotal and seen zero detection, so it's not just avast.
2) The only reason you're getting infected is because you have insecure and out of date software installed, allowing a drive-by download.
Use Secunia Software Inspector to check and update everything that needs updating.
Just come across one of these things while browsing:
Antivirus Version Last Update Result
a-squared 4.5.0.41 2009.10.10 -
AhnLab-V3 5.0.0.2 2009.10.09 -
AntiVir 7.9.1.35 2009.10.09 -
Antiy-AVL 2.0.3.7 2009.10.10 -
Authentium 5.1.2.4 2009.10.10 -
Avast 4.8.1351.0 2009.10.09 -
AVG 8.5.0.420 2009.10.04 -
BitDefender 7.2 2009.10.10 -
CAT-QuickHeal 10.00 2009.10.10 -
ClamAV 0.94.1 2009.10.10 -
Comodo 2556 2009.10.10 -
DrWeb 5.0.0.12182 2009.10.10 -
eSafe 7.0.17.0 2009.10.08 -
eTrust-Vet 35.1.7060 2009.10.09 -
F-Prot 4.5.1.85 2009.10.10 -
F-Secure 8.0.14470.0 2009.10.10 -
Fortinet 3.120.0.0 2009.10.10 -
GData 19 2009.10.10 -
Ikarus T3.1.1.72.0 2009.10.10 -
Jiangmin 11.0.800 2009.10.08 -
K7AntiVirus 7.10.866 2009.10.09 -
Kaspersky 7.0.0.125 2009.10.10 -
McAfee 5766 2009.10.09 -
McAfee+Artemis 5766 2009.10.09 -
McAfee-GW-Edition 6.8.5 2009.10.10 -
Microsoft 1.5101 2009.10.10 -
NOD32 4494 2009.10.09 -
Norman 6.01.09 2009.10.09 -
nProtect 2009.1.8.0 2009.10.10 -
Panda 10.0.2.2 2009.10.09 -
PCTools 4.4.2.0 2009.10.09 -
Prevx 3.0 2009.10.10 Medium Risk Malware Dropper
Rising 21.50.52.00 2009.10.10 -
Sophos 4.45.0 2009.10.10 -
Sunbelt 3.2.1858.2 2009.10.10 Trojan-Downloader.Win32.FraudLoad.fqb (v)
Symantec 1.4.4.12 2009.10.10 -
TheHacker 6.5.0.2.035 2009.10.10 -
TrendMicro 8.950.0.1094 2009.10.10 -
VBA32 3.12.10.11 2009.10.09 -
ViRobot 2009.10.9.1978 2009.10.09 -
VirusBuster 4.6.5.0 2009.10.09 -
Relying on any AV to protect you is not going to be enough.