You can always reinstall the OS. Some professionals do it every year or so, just to keep their systems running.
Most often, it's because you're loading too much "stuff". Is your system tray (bottom right of the screen) filled with icons? (And why does every company think they MUST have an icon in the tray?)
Download and run CCleaner: CCleaner - Home
That will clean out the crap in your temporary directory and other places.
Download and run MalwareBytes: Malwarebytes.org
It will tell you if you're running something that you shouldn't be running. It will also help you fix it.
Get a copy of AutoRuns (from SysInternals - now owned by Microsoft): Autoruns for Windows
Click on the "Logon" tab. Everything there is being launched at boot time. Right-click on the entry and select "Search Online". Your browser will be launched and a search page (often Google) will appear and that file will be searched. Anything that you don't really need (Adobe Reader Speed Launcher, QuickTime task) should be unchecked (you can always check it later if it turns out you need it.
Also, get rid of unneeded "Browser Helper Objects" under the Internet Explorer icon. Uninstall junk you don't need.
The last possibility is that your system is infected with a real nasty -- like a RootKit. There are RootKit identifiers, but they are not easy to use. If you think you might have one, reinstalling the OS is the best way to go.