I run into this type of complaint very often, and it's almost always caused by viruses or spyware. I would recommend that you download two free clean-up tools that are very widely used and trused.
MalwareBytes AntiMalware (MBAM)
(download free version)
Super AntiSpyware Free Edition
(download free edition)
Install both programs using their default settings, run their respective updates from inside the program, and then run the full scans on both. You can run both scans at the same time, they'll both take a long time to run. Expect the scans to take an hour or more depending on how slow your computer is.
When the scans are finished, you'll get a list of "infected items". Follow the instructions for cleaning and deleting them, then reboot your computer. Once it comes back up, you may want to run both scans again just for good measure. If it comes up clean the second time, you should be all better for a while.