Internet will connect via Dial up, but not Cable?!

[ZeroX5]

I installed a cable modem for a friend recently, (I work for a cable company, so it was done correctly). The problem I have is this: When I hooked up the modem (Motorola Surfboard) to their Dell with XP Home, it would not browse the internet in either IE6 or FireFox. The funny thing is though, I can ping any site on the internet via the command prompt, AND if I use their dial up connection, the internet works fine. I can also go inside the modem and look at its specs and such, but if I even try to go on the internet, it gives me a 404 error.

What exactly shows up in IE or Firefox when I use the cable connection, is something about a 404 File Not Found Apache Server thing. The browser will also 'add' an HTTPS:// or index.jsp to whatever I type into the address bar, and try to browse. So lets say I go to www.yahoo.com and hit enter. It automatically becomes https://www.yahoo.com/index.jsp for no reason, then gives that 404 message.

I have tried for spyware, I've ran AdAware on the machine, made sure the updates were current, and used Spyware Blaster. Spyware isn't the culprit here, and I don't know what is.

This isn't the first time I've seen this though. I also saw this on a newer Dell Laptop. It was the SAME exact thing going on in the laptop too. Another thing if it matters, is the laptop and the desktop machines were both using the same Dial Up provider. www.onewest.net

So I am confused as to what this can be. Any help is appreciated.

Thank you.

[glc]

Control panel, Internet options. Connections, LAN settings. Uncheck all 3 boxes. You may have a proxy server enabled.

[ZeroX5]

I wish it were that easy, but there isn't anything checked in any of those parameters. It's very strange....., I haven't found a solution yet....