I hope your happy with MSE until it doesn't detect something as there isn't a single security application that will give 100% protection.
I also hope you get the kind of support that you have got from these forums in the past.
Presumably you have confirmed that detection at VirusTotal.
This detection is in an old backup zip file, which is inert until you extract the file/s in side and try and run any executable at that point the file system shield would scan it. So there was no active password stealer (if the detection is correct) running on your system.
Archive files aren't scanned by default (see image1) for that very reason they are inert, so unless you did a custom scan and opted to scan all packers (image2) avast wouldn't have been scanning, so you would never know if avast would have detected it or not if you hadn't specifically scanned it.