This is an attempt to clear up some confusion on these recent threads:
AVG thread: [webmasterworld.com...]
Trend Micro: [webmasterworld.com...]
Most of the following impose themselves on the SERPs and signal whether or not your link is safe to click - even when they have no information they will flag your site in a way that will discourage visitors, so in this less-than-brave new world it is important to get their seal of approval.
This user-agent is used by Exploit Prevention Labs LinkScanner:
Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1; SV1)
The software was recently acquired by Grisoft AVG but is still available for download on CNET, and if you block it you will be discouraging visitors and filling your logs with 403s for no good reason.
This user-agent is used by Grisoft AVG 8.0 LinkScanner:
Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1;1813)
As above, a bandwidth-wasting (and easily fooled) pre-fetcher best dealt with by cloaking minimal content (example given by jdMorgan in the AVG thread). When I had it blocked I lost a lot of traffic and on one of my tests it produced an impressive 120 (one hundred and twenty) 403s in 12 seconds - without me even visiting the site.
This user-agent is used by Trend Micro Internet Security and TrendProtect:
Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1)
Unlike the others it does not pre-fetch SERPs in real time, but can be triggered from the "website authentication" feature of the Internet Security Pro package on demand, and also appears to be doing some general spidering for the Trend Micro "rating server".
Sometimes it comes from the Trend Micro IP range (188.8.131.52 - 184.108.40.206) but more often it comes from the Japan Network Information Center with various IPs in the 150.70.84.xx range - so you can probably expect your site to be classed as "Suspicious" (the outrageous term they use for unknown sites) if you have APNIC blocked.
This user-agent is used by the DrWeb plugin for Explorer and FireFox:
Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1; SV1; Maxthon)
I have not been able to identify where McAfee SiteAdvisor gets its information, but I do have an amusing screenshot of the related Yahoo SearchScan flagging google.com as a purveyor of "Dangerous Downloads".
On all my sites it says "We've tested millions of sites but haven't tested this one yet" - and unless McAfee scans the entire web as frequently as GoogleBot it is presumably worthless.
None of the other 18 anti-virus packages I tested currently interfere with search results, but it may only be a matter of time, and if you don't appease them and get flagged as "clean" you may find that your ranking is considered irrelevant.
"Paranoia strikes deep - into your SERPS it will creep"