I'm not entirely clear on the network topography you have going on here from your post.
Is it: Cable Modem > Time Capsule > Windows PC (with ICS: Internet Connection Sharing?) > multiple Apple devices?
Or is it: Cable Modem > Time Capsule > Windows PC & multiple Apple devices? (ie. Windows & Apple devices are connected to the Time Capsule? Perhaps you mean the Windows system is connected via Ethernet and the Apple devices connect to the Time Capsule via wireless?)
If it's the first then you are likely actually triple natting things and I'd recomend a setup bypassing the ICS on the windows box, and then can follow one of the lower paths mentioned below.
If it's the other then you're not in as big of a mess and are likely only double natted.
The easiest solution for this 2nd setup would be to go to a single NAT setup by placing your Cable Modem into Bridged Mode. Most Cable/DSL modems are setup to act as routers with DHCP servers built-in. You'd have to look online or on your modem for the specifics on how to accomplish this with your modem.
Another option would be to turn the DHCP server off on the Time Capsule, but make sure to give it a static IP that is on the SAME subnet as the Cable Modem uses (that is not used by the modem itself. Usually 192.168.1.1 or something close.) By doing this the Time Capsule will not do any routing and will function as a switch/Access Point.
The third option would be to leave the setup double natted and simply point the DNS of the Time Capsule to the IP of the Cable Modem. Then any systems that acquire a DHCP address from your Time Capsule will be told to forward DNS queries to the Time Capsule which will then forward them to the Cable Modem, which will then be forwarded to your ISP until the query is resolved. This third may be the simplest solution, but is far from the overall best. However, it should still function.
I should also preface this by saying I do not have a Time Capsule nor have I used one and it is possible it is a firmware issue specific to them. However, in terms of functionality routing devices all do the same basic things at their core.
