First, try rebooting (power it totally off and then back on) see if this resolves it. If you don't reboot or shut down the computer regularly, then things can keep running.
If that doesn't fix it, then try going to "Start" and in the search box type "msconfig" (without the quotes). Launch that program and click on the "Startup" tab. See what is listed as running (which will start as soon as Windows does). Is there anything running that you don't want running?
NOTE: Be really careful messing around in "msconfig". You do not want to mess with the other tabs and don't turn anything off in there unless you know for certain it is safe to do so.
Should none of this work, then run good deep scans for viruses and also malware. Usually you need more than one program for this, but there are many free ones out there.