Did Malwarebytes Anti-Malware and Avast antivirus software not detect the malware you are reporting? Or did they detect it but were unable to remove it? If you have malware on your system, you should remove it first; that may eliminate the problem you are experiencing, especially since you are reporting the presence of the browser hijacker bettersearch.tk on the system. As VirusResearch.org notes in regards to this malware at Remove Bettersearch.tk from Chrome/Firefox/IE:
Bettersearch.tk is an infection identified as a browser hijacker. What
the infections of this type do is to modify browser settings and to
replace the default homepage, new tab, and search engine. These
changes are quite obvious, so it will be easy for the users to
recognize that there is something wrong with their browser. It is
important to remember that when Bettersearch.tk appears on your
browser, you should refrain from surfing the web until the infection
is completely removed. This is due to the fact that the
Bettersearch.tk hijacker will put your virtual security at risk by
presenting you with potentially dangerous search results, ads, and by
doing other malicious activities. If you want to keep your computer
safe and browse the web freely like before the hijacker gained access
to your system, you should remove Bettersearch.tk immediately and be
careful not to let such problems occur again in future.
Take these steps:
- Look for Bettersearch.tk under "Uninstall a Program". If it is there, uninstall it.
- Open Google Chrome, click on the 3 horizontal bars at the upper, right-hand corner of the window and select "More tools", then "Extensions". If you see Bettersearch.tk, get rid of it by clicking on the trash can icon next to it.
- Select "Settings" in Chrome, then click on "Show advanced settings" and then click on the Reset settings button.
Or you can run other antivirus software, which may be needed if it is possible you have other malware as well. I've listed other free antivirus programs you can use to check the system here. Other tools you can use to eliminate this particular malware can be found at the Malware Tips site at Remove BetterSearch.tk hijack (Virus Removal Guide).
If you have taken all of the steps you listed in your post, then it would seem your system is being reinfected, perhaps because you are visiting a website or websites that are reinstalling the malware or you are reinstalling a program that carries malware with it after reinstalling the operating system. If you are reinstalling software that may possibly carry carry malware within itself or install additional software when it is installed, it would be advisable to scan it with antivirus software before installing it. You may be able to upload such software first to a site like Google's VirusTotal site that will scan files with multiple antivirus programs for free. VirusTotal scans files with 56 antivirus programs currently. Or you may be attaching a USB device that is infected which reinfects the system when you reattach the device, if you don't have antivirus software on the system protecting it from such events. E.g., see the malware infections section of the Wikipedia article on USB flash drive security
Make sure you keep the operating system up-to-date with patches from Microsoft and keep antivirus software on the system that performs real-time scanning up-to-date. Free versions of some antivirus products will only scan on-demand, i.e., only when you manually start a scan. So, if you have no real-time scanning antivirus software you may disinfect your system, but then have it reinfected again and have to repeat the cycle.
