You will want to start looking for "back doors" that are possibly being used by outside compuetrs. If your computer (or internet connection) is suddenly active and you're not doing anything then this should be a red flag. You can monitor the activities on your computer to some extent with the WIndows Task Manager (Ctrl+Alt+Delete to open it). The problem with using Windows Task Manager is if you have a rootkit on the computer, it can hook the DLL controlling Taks Manager and make it "lie" to you; it's self preservation.
First, take a look at your Task Manager, note any processes which you feel may be odd and post them in your next response. Afte rthat I would suggest using Rkill to stop any rootkit processes that may be starting when the computer comes on. Then I would run the Malwarebytes and SUPERAntiSpyware. Here are some DL links for the Rkill....LINK 1LINK 2LINK 3LINK 4
Save it to your desktop and then double click to launch it (With Vista you need to right click and select run as administrator). You should see a little black window open and then close. If you see that box then it worked. If you don't see the black box then delete the file and use another download link and repeat the steps.
Once it runs you should be able to install, update and run Malwarebytes. You can get it HERE
. Install, update and run it. If you have problems installing it then try renaming the file before saving it to your desktop. I like renaming it to bgmama. Get rid of what it finds.
Remove what MBAM finds and then install and then run ATF Cleaner
(for WINdows XP and 2000 only!). Check the box for select all and then click the button Empty Selected. This will help clear out temp files, cookies and other junk that clutters up Windows.
Next install, update and run SUPERAntiSpyware
. Get rid of what it finds. Finally update and run your antivirus program and get rid of anything that it finds.