Thanks again DavidR and thanks polonus
Regarding online scans. Since no av is 100% and can have blind spots like the rest of us, usual recs [in articles I've read] have been to "occasionally" scan w/ another av. Since you can't install 2 av's at a time [not even if you keep one disabled], we paranoids who only sorta know what we are doing use an online scanner at times to get another av's "opinion" w/out having to uninstall and reinstall our av. Especially when we paranoids were stuck using symantec av, which didn't inspire us w/ confidence. I don't feel the need to do "occasional" scans now that I'm running avast. I'm also re-thinking what/if any anti-malware programs I need to run regularly.
Of course, one should only use scanners from well-known companies like trend or panda [I didn't know about avast then so I don't know if avast has one]. I think the recs need to be a bit more specific about how to choose a 2nd av for online or "on demand" scanning to compliment your installed av. That would require constantly updating info on which av's are catching/missing x% of likely/possible threats and maybe comparing heuristic vs definition strength and posting it where I can find it with my google searches. Probably not realistically doable. No article said how often "occasionally" was either.
I haven't done an online scan for awhile for long story reasons. You used to run the online scan from within the browser and they used to tell you to turn off installed protection during the scan, so you just went to that site and didn't do anything until the scan was over. With the recent trend housecall scan I downloaded a launch.exe via Firefox [ I don't use IE x when MS makes me and only at MS] that made a web connection to update when I ran it. It then runs a stand-alone, browser independent scan. I downloaded the symantec scanner [by mistake] and it also runs outside the browser [and produces uninterpretable results] so I was thinking that's how "online" scanners work these days. Trend said somewhere its scanner worked w/ installed av and antimalware programs in place [as I understood the wording]. I stopped my AVG real-time and Spybots teatimer 'cause I wasn't 100% sure of the wording and I didn't want any false positives or whatever else could happen.
AV products, the type/prevalence/effectiveness of online threats, and other technology that impacts on security change constantly and there may be times when it's better to run a 2nd scan and times when it's unnecessary or it's better not to run a 2nd scan and only the experts will probably know. I think the current recs are based on picking an online scanner provided by trend or panda or another such av which has maintained a good rep over time as your on demand scanner. If your avg, bitdefender or symantec missed something, they have a good chance of finding it.
One circumstance where I think it might be a good idea to run a 2nd scan is when you run into a troubleshooting issue that you can't fix easily and you get a lot of possible causes/solutions suggestions in forums or reading. I did the trend scan because I'm trying to figure out why this computer has run way too slow out of the box and why MS Word crashes constantly on MS WinXP. Since the computer has been in use w/ various av's and antimalware programs at various times I wanted to first rule out malware that I may have picked up to confuse the issue with as close to 100% confidence as I could, then do hardware diagnostics, then performance diagnostics. The Trend scan found malware that my then-av hadn't and I found out about rootkits, so I haven't progressed to hardware diagnostics yet.
Re secure downloading. Prior to installing avast, I used to scan any downloaded file before opening w/ my av and spybot. [via rclick or context menu] With spybot, when you try to scan some .exe files from the context menu sometimes you get the "Do you want to run this" box instead of a scan, but I haven't had that problem w/ malwarebytes. I also use macafee software site advisor before downloading software from a new site [unless it's well known like sysinternals or cnet or pcworld] I've haven't routinely exported my registry before installations, which is a good idea. I only tried System Restore once and it didn't work.
With intelligent stream scanning I thought maybe I wouldn't have to scan pdfs & pics after download before opening.