I have a laptop with Vista Home Premium and Express card slot.
As per my recent direct interaction with AirTel's technical guy, NO data card (slot and usb) available in market as on date from them in India is compatible with Vista and Express Slot type.
If somebody has a laptop/notebook with Vista (any generation) and Express type card slot, DO NOT go for any currently available data card from AirTel and simply wait until further official communication from them. The tech guy clearly told me that the Express type AirTel cards will NOT come in near future whereas the USB-mount Vista-compatible AirTel cards may come in market by early October 07.
So folks, if you are seriously interested like me in AirTel data cards to use on your Express/Visa combi laptop, pl keep your head cool, wait till mid October and obviously continue with wired broadband.