Windows Defender is just fine (especially the W10 edition) and more than capable. I use it on all my systems and have for years without incident. I would not suggest it if it was not capable.
To suggest it is not reliable is just plain silly as that would mean there are 100s of millions of users who are infected and that is just not happening.
Remember, what incentive does Avira, AVG, Avast, McAfee, Norton, Kaspersky, and all the others (EXCEPT Windows Defender) in the antimalware industry have to rid the world of malware? Absolutely none whatsoever as that will put them out of business.
Microsoft, however, has all the incentive in the world because otherwise, the naïve, the IT Press, and the MS bashers will blame Microsoft anyway even though it is the bad guys perpetrating the offenses and the antimalware industry that failed to stop malware in the first place.
The only incentive all those other players have is to coerce users into paying for their pro versions.
Regardless your primary scanner of choice, you should have a secondary scanner just to make sure you, the user and always weakest link, didn't let something slip by. I recommend and use Malwarebytes's Anti-Malware
(MBAM) for that. I have been using MSE then WD since W7 came out in 2009 and so far, MBAM has not found anything malicious WD let slip by.
Note too Microsoft does not code WD to earn top scores on synthetic lab comparison tests. Instead, it codes WD to thwart current real-world threats and it does that very well.
That said, the best defense is to keep Windows fully updated and avoid risky behavior like visiting illegal gambling and pornography sites, participating in illegal file sharing of tunes and movies via P2P and Torrents, or being "click-happy" on unsolicited downloads, popups, links, and attachments - things you need to avoid regardless your security setup.
I haven't downloaded anything onto my laptop lol only a couple of programs which are 100% clean like Team Speak and Audacity.
I highly doubt that it has anything to do with a virus or something.
You don't have to consciously download a program to become infected. That said, if you did not disable Windows Defender, kept Windows updated and are not the "click-happy" type, odds are it is not malware or else this would affect much more than just PayPal.
This really is something to fix however rather than giving up. Did you cleanout your cookies with Windows Disk Cleanup or CCleaner?
Can you bring in a third computer and see if they can reach PayPal from your network? Or take you notebook to a friend's network and see if you can reach PayPal from there?