I'm not going to give you some 'run this spyware cleaner, run that virus cleaner' solution. These tools below are the ones that truly help you find what's wrong so you can move forward w/ ammunition.
Download Process Explorer and Autoruns from
Microsoft Sysinternals 1st, open Process Explorer and look at the processes, particularly paying attention to the Company Name field. If any are non-Microsoft and you don't know what they are, right click on the process and do an online search to see what it is.
Next, run Autoruns. Once it opens completely go to Options and checkbox the Hide Microsoft Entries. Then refresh it again and see if there are any startups that look suspicious. Let us know how that works.