Do you know anyone who can bring round an alternative ADSL modem for you to try? Have you tried uninstalling and then reinstalling the drivers that came with your modem card?
Or have you contacted your ADSL ISP to ask for their help and assistance. After all, if you're paying for the service they're honour bound to at least offer some advice.
I have managed to get my broadband link from both Win98 and WinXP PCs with surprisingly little difficulty, and getting my PowerMac linked up was a doddle.