Sounds like you have somthing in your startup folder or registry.
First check the startup folder by clicking Start>Program files>Startup and have a look to see if there is anything in there that shouldnt be.
You can also run msconfig and then click on the startup tab to see what processes are being started when the pc boots up. Again, look for anything that you dont think should be in there. If you are not sure about something just do a quick search for it on google and you should soon be able to find if it should be there.