|"I do have an antivirus program, Bitdefender. I paid 50 euros for it and it shouldn't be outdated yet"|
50 euros? There are so many good free anti-virus & anti-spyware programs available, I don't know why anyone would pay for one.
Your system appears to be infected, regardless of your daily scans. Could be adware/spyware/malware, possibly trojans.
Other than installing & running Malwarebytes (which you should definitely do), here are a few things I suggest you check 1st. Start by pressing Ctrl-Alt-Del to bring up Windows Task Manager. In the lower left corner, how many Processes do you have running? Just to give you an idea, I have 38. Then click on the Processes tab...at the top you'll see the column headings (Image Name, User Name, CPU, etc). Click on "User Name" to sort the list, you may have to click twice. Now look at all the processes that have your User Name next to them...you *should* be able to identify all of them. I have 10 entries & know what each one of them is. If any entry looks suspicious, try googling the name to see what you come up with.
While still in Task Manager, Processes tab, click on "CPU" to sort the processes by CPU usage. Once again, you may have to click twice. Which process is hogging the CPU?
Next, download, install & run CCleaner Slim...both the cleaner & the registry scan. Remove everything it finds.
You can also use CCleaner to check which programs load at startup. Click on the Tools button on the left side of the CCleaner window, then click the Startup button. See anything suspicious?