Cable modem - PC - Airport Express - MacBook Pro

Hi, I just bought AE hoping it would help me wirelessly connect to internet with my Mac laptop. This doesn't quite work for the reasons I'm trying to find out..

My cable modem is connected to PC by coaxial cable. Then, I connected Airport Express to it (the modem) via ethernet cable. Is this setting possible at all or do I need a router to make it happen?

I appreciate your help.

Most of the times, with your internetconnection, you get a modem from your provider, which you NEED to use, cause this is kinda your keychain to the internet. from this point on, you can connect it to the PC, as long as it is gives away the connection again.

My advise is:
COAX -> Modem -> AE -> everywhere

Thank you for your response!

I'm sorry, I misstyped...

Cable modem (motorola SB5120) is connected to PC by USB cable. Internet connection is working.

Then I connected AE to the modem (still connected to PC) via ethernet cable. The thing I'd like to know is can I have internet on both PC (connected to the modem via usb) and MacBook (wireless through AE), or do I need another router, base..?

For as far as i know, should be no problem.

Thank you, it's a relief to know I'm not gonna have to return AE back.

Now...
When I connect AE to the modem some weird stuff starts to happen.
At first, light on the AE is green, but then turns flashing amber.. Then my cable modem starts restarting over and over again, flashing SEND and RECIEVE lights only, right before it restarts. This, of corse, results in losing internet connection on my PC.

AirPort utility on PC doesn't seem to detect AE either (it IS detected by MacBook, but naturaly there's no internet connection), but this is still not an issue since I have to solve the first problem first.
At first I thought the modem couldn't handle both PC and AE wired on it but you told me that was most likely not a case. I don't know where to start with this one.