New iMac, cannot connect to internet

I persuaded a colleague to switch to a mac and sure enough my reward is that he cannot connect it to his existing wireless network.

He is using a SpeedTouch adsl modem/wireless router as supplied by SingTel (we are in Singapore). He has two Windows laptops both of which connect happily to the router and access teh internet.

When he turns on Airport on the iMac it finds the router and he has put in the correct password. It shows the bars on the airport icon but has an exclamation mark in the middle.

It cannot connect to the internet, however the windows machines continue to be able to do so.

He is obviously fast losing faith in my assurances that this is a better machine. He spent two hours on the phone with a Singtel helpdesk person and that guy was unable to resolve it. He confirms the settings of the modem/router are all correct for accessing the SingTel internet connection and suggest, naturally, that it is an Apple problem.

Does anyone know of any issues that the new (21.5") iMac is likely to have with an existing modem/router that was originally set up for a Windows machine and that continues to work fine for a Windows machine?

- type of security password (eg WEP vs WPA/WPA2)???
- channel?
- anything else known to be an issue?

Thanks in advance

Martin