Program removal

I've recently changed from BT to a new ISP. I want to remove all the BT
program from my computer, I went into add & remove programs, located the BT
Broadband program and removed it. However, when I use Windows Explorer I can
still see the program in 'Program Files'. There is a uninstall icon but this
just tells me to go to add & remove programs. I've tried deleting the program
from Explorer but it won't let me delete some files. How do I get rid of BT
from my computer please as it's taking up memory which I'm short off?

Thanks

Rod