Run a disc defragment, Clear internet cache, see if your hard drive is full (if yes, clear it and delete unused files), run a full Antivirus test an all drives (if you dont have one, this is a good free antivirus http://www.avast.com)

i will suggest format your hard drive, not quick format, normal format. format only the drive with operating system in it. save your data on some other drive. install a new operating system. after that install a good antivirus, i suggest avg free. thats it. turn off automatic updates. and boom you are done.