I have faced an interesting issue recently, my browser was hi-jacked. This is the latest fed on the internet - hackers write software that forces one's browser to go through a third party website to get to the page you want to go. E.g. you type in
https://www.yahoo.com and you do get to Yahoo, except there are some buttons on top or bottom of your browser and other strange things. This can be annoying and at the same time dangerous. You never know if there is key logging and people will break into your bank account/paypal account or just email account.
In any case, here is when happens:
1. You visit some borderworld webpage
2. Download some third party free software: toolbar for the browser from an unknown company, etc (I got mine through an extenstion of MSN Messenger, called MSN Plus; that little piece of crap installed a ton of stuff).
Why they do it? Advertising, popups, money
For me tt was very very annoying. Any time I would open my broser, I would get a popup or some weird message. My homepage was set to something like this:
https://searchnow.com?passthrough=https://www.google.com. (Google was my original homepage). I would change the homepage setting every time and every time it would be back to this crap. I tried searching my HDD, etc, but no result. Two days I was fighting and then went online and found that the syndrome is very common and there are tools for it.
There are a lot of tools available here:
https://www.spychecker.com/software/antispy.html
You may be interested in installing some of them; my favorite tool is "Hijack This"

it is a simple scanner and fixer, but you have power what gets zapped. Good for once a week check, not constant monitoring.
Another good resource is Norton Antivirus, which right now is Free on Amazon.com after 2 $20 rebates (MSRP $40)
<A HREF="https://www.amazon.com/exec/obidos/redirect?tag=gmatclub0c-20&path=tg/detail/-/B0000C8XYI/qid%3D1085180524/sr%3D8-1">Norton Antivirus 2004</A>
The best, however, is Norton Antivirus 2004 Professional - it has a built in spyware component. It is $59 on amazon, but can be downloaded for a free trial here:
https://www.soft32.com/download_18175.html