That really shouldn't happen
This is usually done by a script which is definately not meant to be on as default. Do you have AV etc installed, as my first guess as to why this has happened is that you have been infected.
Hopefully removal of the virus will cure it, otherwise you have to find the script and remove it.
Try running msconfig and in the startup tabbed disable verything which doesnt look necessary.
Also, try running in safe mode and see if it still happens.
Let me know how you get on.
crabby