Since the introduction of Windows Vista the networking software was completely rewritten by Microsoft (in previous versions of Windows they were 'borrowing' that kind of code from BSD). Therefore this code will be included in Win7 as well.
Having first hand experience with setting up a wireless network using Vista (without SP1) I can tell you that I was (very) badly impressed with the stability of the wireless connection, while my own laptop (XP) was running fine in the same network.
To me it appears that the new network software is not only more extensive but also more 'sensitive' than the old software, although I hear that Win7 has improved in that field.
Now I don't have a clue how your router has been setup, but when setting up my ex-girlfriend network I enabled the WPA, limited the amount of DHCP users able to connect to this router to the absolute minimum (in her case 1) and enabled the MAC address filtering.
Mac filtering is not that useful because it takes hardly any effort to find software that will change this for you. Come to think of it, maybe that kind of software is able to help you getting rid of it (in the case of a false entry) or at least point you in the right direction. Here are some links
SMAC,
MACmakeup,
Technitium MAC Address Changer or see
here how to do it manually.