Summarize this article in one sentence.
If you aren't running any programs and your hard drive light is constantly turning on and off, or you can hear the hard drive working, you may have a virus that is working in the background. If you start noticing that your computer takes significantly more time than usual to start, a virus may be slowing down the startup process. If you can't log in to Windows, even with the correct log in information, a virus has most likely taken over the log in process. If you don't have any programs running and your modem transfer lights are constantly blinking, you may have a virus that is transmitting data over the network.
Check your hard drive activity. Time how long it takes your computer to boot up. Look at your modem lights.