Have you run any installers of late? Some of them are known to change user settings.

FredMcD said

Have you run any installers of late? Some of them are known to change user settings.

Could you please give me some examples of installers (both the regular kind and the kind which change user settings)?

Regardless of how that Malware got onto your computer, you need to get rid of it.

You can try these free programs to scan for malware, which work with your existing antivirus software. You should run then all, as many times they each look for different types of Malware than others may look for:

• Microsoft Safety Scanner
• MalwareBytes' Anti-Malware
• Anti-Rootkit Utility - TDSSKiller
• AdwCleaner (for more info, see this alternate AdwCleaner download page)
• Hitman Pro
• ESET Online Scanner

Microsoft Security Essentials is a good permanent antivirus for Windows 7/Vista if you don't already have one. Windows 8/10 have antivirus protection built-in.

Further information can be found in the Troubleshoot Firefox issues caused by malware article.

Did this fix your problems? Please report back to us!

It’s very sad, but many of the software downloaders / installers will trick you into installing not only their program, but other programs as well. You have heard of the fine print in shady contracts, right? Well, some installers you need to look at the itsy bitsy teeny weeny fine print. You are thinking you are giving the installer permission to install the program you want by using the recommended option. But if you use the Manual Option Instead, you discover all kinds of stuff that you do not even know what it is or what it does. From now on, everyone needs to Use The Manual Option to put a stop to this.

the-edmeister said

Regardless of how that Malware got onto your computer, you need to get rid of it. You can try these free programs to scan for malware, which work with your existing antivirus software. You should run then all, as many times they each look for different types of Malware than others may look for: Did this fix your problems? Please report back to us!

I've already run 99% of the programs you suggested and they found nothing.

FredMcD said

It’s very sad, but many of the software downloaders / installers will trick you into installing not only their program, but other programs as well. You have heard of the fine print in shady contracts, right? Well, some installers you need to look at the itsy bitsy teeny weeny fine print. You are thinking you are giving the installer permission to install the program you want by using the recommended option. But if you use the Manual Option Instead, you discover all kinds of stuff that you do not even know what it is or what it does. From now on, everyone needs to Use The Manual Option to put a stop to this.

Thanks. I did have to reinstall Avast antivirus recently, so I'll look into that sometime after Thanksgiving.