I'm not entirely confident that the boot-time scan would be able to extract the suspect file from within the .msi file.
Not so much a relatively new 'virus' (wrong term this isn't a virus) but a trojan detected using a generic signature. The avast Win32:Trojan-gen is generic signature (the -gen at the end of the malware name), so that is trying to catch multiple variants of the same type of malware and is a fine balance between detecting a new variant and detecting something valid as infected.
Personally I'm unsure of this one, considering some of these are supposedly from google, but there is no easy way to extract the suspect file from within the :\WINDOWS\Installer\4eb8a4c.msi file, I do also get the shivers when I see these really meaningful names (NOT) as malware commonly uses randomly generated file names.
You could try to upload the :\WINDOWS\Installer\4eb8a4c.msi file to virustotal for scanning if it isn't too big (how big is it) as there is a 10MB limit.
You could also check the offending/suspect file at:
VirusTotal - Multi engine on-line virus scanner and
report the findings here the URL in the Address bar of the VT results page. You can't do this with the file securely in the chest, you need to extract it to a temporary (not original) location first, see below.
Create a folder called Suspect in the C:\ drive, e.g. C:\Suspect. Now exclude that folder in the
Standard Shield, Customize, Advanced, Add, type (or copy and paste) C:\Suspect\* That will stop the standard shield scanning any file you put in that folder. You should now be able to export any file in the chest to this folder and upload it to VirusTotal without avast alerting.
If it is indeed a false positive, see
http://forum.avast.com/index.php?topic=34950.msg293451#msg293451, how to report it to avast! and what to do to exclude them until the problem is corrected.
