Is that different from system restore?
I think DEFRON is implying that you should have a backup.

I agree, but if you don't have one then that's obviously not an option.
If you primarily use Windows I'm a big fan of http://www.macrium.com/ for scheduled backups (it's also a good imaging tool).
It can also be the sites you are visiting. Sometimes they have an ad that will direct you to that scanning website.
It happens occasionally to me, but only with a handful of websites.
It's possible, but this behavior is usually the result of some kind of malware that needs to be removed. Browser exploits and trojans are the usual suspects.