Bandwidth and traffic limitations

I need to limit availiable bandwidth and traffic on our connection to Internet basing on users’ accounts (for example – no more than 1024kbytes/sec and no more than 100Mbytes/month for user).
I use Windows 2000 Advanced Server and NAT to organize access to Internet.
Can anyone recommend me software product to solve this problem?

Sincerely,
Cyril Sluchanko