Weird problem in sharing Cable/modem under XP Pro

[zener]

I share my cable modem via netgear wireless/router MR814 between two PC using cat5e connections ( no wireless). One PC with single user setup, give me 2-2.2 MBits transfer rate. The other, with multi user setup, always give me 0.55 Mbits transfer rate when login under any user, including admin. Is the transfer rate restricted somewhere by Xp under multi user setup? Thanks!