Hmm so you already resetted the modem... supposedly by unplugging it [if not try that].

If the ISP sees your proper IP [although you shouldn't get new IPs on cable in most parts that I know] then the only thing I can htink about is connection hijack, so I would do a traceroute ASAP and see what awkward IPs that gives you.

Alternatively you could try downloading something like Knoppix LiveCD and see how that works [ifconfig would give you your IP] and in case you get the proper one then I assume something's wrong with Win. Run an AV program, Google for that HJT output... If however you get the same ip then either your ISP cache is at fault or... something else that I didn't think of.

I would also ping the IP you should have and the one you do have and see how that works, eventually try and traceroute those aswell... see to whom the IP belongs to [if it's not your ISP] and if the problem persists send them an e-mail.

Keep us posted...