721 error

Hi all

i've been using bt broadband for 18 months with the alcatel speed touch external modem. 10 days ago i've decided to reinstall my win xp operating system to clean up my hard drive and after with i am not able to connect to internet i get 721 error all the time i phoned bt to solve this problem and they sent some body to check my phone line and modem, everything is working perfect he connected to internet with his laptop with my modem and user name but some thing wrong with my computer actually it happend about six months ago again whaen i reinstall xp and i had a same problem about 1 week and i solved the problem now i can't remember how i did

i am using dial up connection now but i can't use the broadband

thanks guys