Came across an odd problem today – the PC was able to access normal websites, but not https secure sites? The user reported having problems with an uninstall of Norton (they were now using AVG). After a bit of hunting on google, I found that this was indeed due to an incomplete uninstall of Norton (in this case Norton 360 – however problems with other versions were reported too).
The solution? Symantec provide a free tool for cleanly uninstalling their AV tools:
http://service1.symantec.com/Support/tsgeninfo.nsf/docid/2005033108162039
After running the tool, the problem with https sites was resolved!