I set my preferences for Boinc to access internet only after asking. However it is continuously dialing up (modem) and disrupting my other programmes running. I have had to uninstall Boinc to prevent this. Any polite answers please?

What do your General Preferences say for the line:
Confirm before connecting to Internet?

If you want it to NOT dial-up automatically, you need to set that to "YES". That will make it ask you before it tries to dial in.

If yours is already set to "YES", then I guess that feature doesn't work as it is intended.

OR, maybe your Windows settings are overriding it. If you are NOT connected to the Internet, and you open a browser and try to go to a page on the Internet, does it just start dialing? Or does it ask if you want to connect?

This is a known issue. Try disabling BOINCs network access on the file menu of the GUI.

For this look also at the following thread:

http://setiweb.ssl.berkeley.edu/forum_thread.php?id=480