im not an expert or anything and this probably wont help but ill post just incase.
I recently also bought a new pc (from cube247.com) with vista premien installed.
When i plugged my cables back in for the internet i also could not access the internet on the new computer.
I got a guy round (computer ads in local papers) and he said windows wasnt activated on my computer. He got into my Bios on my computer and turned it on. Still though i had trouble. I then tried with a router and the internet worked fine then.
However, using a router, although the internet worked fine, i started getting massive packet loss on online gaming and he suggested i change my ethernet lead to a cat6 one.
I did this, changed the lead to a cat6 ethernet cable and even had virgin round to re-route my broadband as i was setting up the new computer in a different room.
After i had it re-routed, and plugged a cat6 0.5m ethernet cable between the modem and my computer, it started working fine, no need for a modem, and its worked fine since.
Before i did all this i printed off the broadband installation guide on virgin media's website for windows vista and it says this:
step1. plug one end of your ethernet cable into the back of your cable modem, and the other end into the back of the pc. then esnure the power cable and coaxial cable are connected to your cable modem and switch the power on. (note have pc off at this point).
step 2. Turn on PC.
Step 3. Next double click on Internet Explorer on your desktop oe click START>INTERNET EXPLORER. Youll need to type
https://autoreg.autoregister.net into your adress bar and then hit enter. Now select the "broadband" button.
Step4. Once youve done this, follow the on screen instructions to set up your account. This will inlude creating an email address, password and security question.
You need to have on hand the letter we sent you with your PIN, surname and serial number. This was sent to you separatly. If you dont have the letter then ring us on 151 from a virgin phone or 0845 454 1111 from any other phone.
Anyhow, that above didnt help me at the time as i couldnt get a connection but it doesnt hurt to post just incase it works for you.
I think i just got lucky with mine, and hope you resolve yours soon.
lee