^^Yea..its surely the Mac address which is locked up at server..Heres how to do it:
On your laptop:
run "cmd"
and
execute "ipconfig /all"
see the physical address of your ethernet adapter (on which pppoe conection works)
It should be something like XX-XX-XX-XX-XX-XX
Keep this in mind (or write it down somewhere

)
On your pc:
Goto Network Connections -> Right Click on your LAN Connection (on which pppoe works) -> Properties -> Configure -> Advanced -> Network Address
It is by default "Not Present"..Click on Value and put the value you got from your laptop without the hyphens (i.e XXXXXXXXXXXX)
Prolly you should restart your comp and now when you connect, it works like a charm