internet connection

I can connect to the internet if I connect the cable modem directly to the computer.
I cannot connect if I do it through the router, even though it says I am connected.
Generic Realtek router.
I actually was able to connect once, but am unable to do it again.
I would be willing to replace the router if necessary, if someone could tell me what to get that would work reliably.
Using Vista 64 bit.
Thanks so much,
JR
ps i've looked thru the ng in detail, tried the fixes mentioned, but to no avail. thanks again. jr