pppoa authentication problem

after a migration from one provider to another (onetel to boltblue in
UK) I experience very curious effects:
3 different ways to connect

1/ win xp and fujitsu usb ADSL modem --> authentication last forever =
no connection

2/ win 98 and same modem it works..... ! why???

3/ any OS and ethernet modem/router conexant based pppoa
authentication last forever (log file and status shows adsl line is
open but ppp status shows not connected)

noway to speak with someone who knows what he is talking about...
downgrade to win 98 is unaceptable and I feel ripped off

but... before anything else I do want to understand what causes this
weird situation...

of course both router and modem have been tested with other
account/PCs

any help welcome...
thanks a milion