Lenovo pre-installed the dangerous Superfish adware on many of its machines, leaving users vulnerable to "man-in-the-middle" attacks. I'll tell you how to find out whether your machine has it, and how to uninstall it.
Superfish is bad news for several reasons. When you visit Web sites, it injects ads into them, so you see not just ads on those sites, but ads that Superfish sends you as well. Even worse, it leaves you vulnerable to man-in-the-middle attacks in which you appear to be visiting a safe, secure site, but you're in fact visiting one controlled by an attacker. Check here for more details.
So what can you do? First, check whether your Lenovo machine has Superfish on it. Lenovo claims that it put Superfish on "some consumer notebook products shipped in a short window between September and December" of 2014, and on no others. But even if you have a machine shipped outside that window, it's a good idea to make sure it's not on your machine.
Head to this Web site. It will check whether you've got Superfish.
What if you've got it and want to remove it? You can do that using Windows' certificate manager. First press the Windows Key + R to open the Windows Run box. Then type certmgr.msc into it and click OK. You'll bring up the certificate manager. Select Trusted root certificate authorities --->Certificates in the left-hand pane. Scroll down the list of certificates until you come to the Superfish entry. Then right-click it and select Delete. You should also run your anti-virus software, because that may catch it as well.