When the connection drops can you log onto/ping the modem (192.168.100.1)? Can you ping the default gateway - the one from the router - should be similar to the WAN IP but ending with a 1?
Does the router still think it is connected?
Guessing you have an ambit modem - do either the sync or ready lights start flashing when the connection drops?
Before you start looking for run on the start menu it isn't there on Vista by default. Try Start->All Programs->Accessories->Command Prompt From there you can run some ping tests and traceroutes to try to establish where the connection is dropping. just type teach line below and press enter - copy and paste the results here when the connection comes back up.
Code:
ping 192.168.1.1
ping 192.168.100.1
ping (whatever the default gateway is)
tracert bbc.co.uk
tracert 212.58.224.131
Oh, and just thought, have you tried it wired to the router? Does that have the same issue?