In reply to Timmd:
Well tbh the slightly faux-naïve tone of the OP and the description of the unusual - not to say apparently unique - phenomenon does rather raise a suspicion of trolling.
In answer to the OP: no, it is not normal for websites to suddenly appear on your computer. AS was (slightly flippantly) suggested, could you have accessed them late at night, but forgotten? Or does anyone else have access to your computer?
Otherwise chances are you've picked up some rather nasty malware, so you will have to download anti-malware software. Or call your friendly local PC advisor and pay them to do it.