#1. Have you recently added any new programs/software.
#2. Have you tried to do a system restore to a point prior to the problem starting.
#3. Have you done a boot scan since the problem started with Avast or any other AV program.
Some viruses can disable your AV. The link below will take you to Avast it is free I suggest you DL it and do a boot scan. When you install it , It will ask you if you want to schedule a boot scan select yes.
http://www.avast.com/eng/down_pro.html
You can try BearShare Lite you can get it at the link below.
http://www.bearshare.com/lite/