While I was reading a news article on CNET (I use the latest version of Pale Moon as my default browser) I noticed a pop up from Avira saying it had blocked access to the registry. There's no other info it provides, just that it prevented access.
I don't recall ever having anything like this happen before, at least not seemingly out of the blue or without manually editing the registry myself. My first thought was a drive-by download from a hijacked page but the extensions I use (uBlock Origin, uMatrix, and httpseverywhere) should theoretically prevent such things. While I can't remember exactly what other websites I may have had open I'm pretty sure I didn't visit any place unusual or out of the ordinary that I don't check most everyday without problem.
Other than that nothing out of the ordinary has happened since. I disconnected my PC from the Internet for a time anyway.
Things that might have caused it:
Shortly before this event happened I had created a Windows XP Home edition bootable USB using Rufus (specifically the portable edition which I ran from the desktop).
A few days earlier I had installed a few things - Microsoft .NET Framework 4.5.2, ScpToolkit, Microsoft Xbox 360 Accessories 1.2, and received an update for Pale Moon 26.2.0 (x86 en-US).
ScpToolkit might have something to do with it as I believe it has some automatic update features and the program itself deals with drivers and the registry.
I am using Windows 7 Ultimate 32-bit.
My anti-virus (Avira and Malwarebytes - both the free editions) is always kept up to date to the best of my abilities. I ran some scans with both of them but came up with nothing. Still, I'm feeling anxious about the whole thing and would like some peace of mind, if you can provide it.