Why do you keep getting it? Can't really say, but it is downloaded from the internet so that probably means some scam web site or other. If it is allowed to download thru a browser then it is halfway there, because most anti-viruses do not check what is going on inside a browser session. If they did, browsing would drag down to a crawl because the AV would be scanning every single element of any web page.
It's scareware. The Window you see tells you your computer is infected and displays how many infections there are. That's total lies of course but the window offers to clean your system if you purchase their products.
I don't get them, but I use a non-IE browser, Firefox, and it is armed to the teeth with security add-ons like WOT, (Web of Trust), which tells me if any web site I am visiting, or intend to visit from a search engine, is considered dangerous. I also have NoScript, which disables all scripts until I allow them. IE doesn't have such add-ons.
What to do:
1] In Safe Mode, (F8 on startup), run your anti-virus.
2] On this computer, download and install both Malwarebytes' Anti-malware and SUPERAntiSpyware. One at a time starting with MBAM, run each, update their definitions, then run a full scan.
3] If this virus doesn't allow you to use both MBAM and SAS in Normal Mode, try them both in Safe Mode. If you are unable to update either, run just MBAM in safe mode because it's installer file should have installed sufficient definitions to perform an initial scan and remove this.
4] If this virus doesn't allow you to download MBAM and SAS, use another computer, (relatives, friends, etc), to download both the MBAM and SAS installer files, then copy them to some external media like a CD or memory stick. At the same time, download MBAM's definitions from http://data.mbamupdates.com/tools/mbam-rules.exe and copy them to the media. Then use that to copy to your computer.
5] In some instances malware will try and prevent known anti-malware scanners from installing. In those cases, before copying MBAM to media, rename the installer file to some other name, eg "GoGetum.exe". Copy the file to the infected machine and run it to install MBAM. You may have to rename the MBAM executable application file to some other name before running it. Then use the updater file to update MBAM then run the scanner.
6] You 'may' have to run the MBAM scanner from Safe Mode.
7] After scanning with MBAM, then run SAS.