There is a chipset that does not get on too well with certain Broadband modems and can cause a conflict, unfortunately I'm not sure of the two items that cause this conflict, but other members of the forum do and will inform you. j.
I was told by AOL tech support that chipsets sis7001 has known issues with usb modems but i run 1mb broadband with this chipset no problems. If you have problems you only have to put in a pci usb card with a different chipset.
I think the conflict you are referring to is between ViA chipsets which according to this link click here is used in the Asus A7V motherboard, and Alcatel USB modems. The link suggests possible solutions. To avoid possible conflicts you may be better connecting using an alternative method, PCI modem or router.
KT133 is also mentioned in the link I gave you, I don't know if it's the same as KT133A but if you are definitely wanting to use a USB modem for your ADSL connection, I think your best bet would be to avoid Alcatel(now Thomson) modems.
If it's BT Broadband (rather than BT Yahoo) that you are considering, the modem they bundle with the connection does not appear to be an Alcatel modem so you should be OK. (BT Yahoo on the other hand are offering the Alcatel Speedtouch 330 which you would have problems with). Like you say, you are probably better off checking with your chosen ISP to see if the modem included is the Alcatel. You could always accept the Alcatel modem, buy another and flog the Alcatel on Ebay...you wouldn't be the first!