I've just read this in your post:
"My username and password are set on the system . . . hence the resaon i stated that i'm 100 % certain they there has been no error"
You've certainly got more faith in Windows than I have! Windows has a nasty habit of 'screwing up' the information in its dial-up settings. Before spending money on contacting Virgin, try checking that the settings haven't mysteriously changed themselves. (It's happened to me several times and I've also had to deal with the same problem on friends' computers):
Go to Dial-up Networking and right-click on your connection to Virgin. Select 'Properties' and click the 'Security' tab. Check that your username is correct, then delete and retype your (asterisked out) password.
That might well fix the problem. However, there have been occasions when (even though the visible evidence suggests that everything is OK) I've found it necessary to right-click on the connection, select 'Delete' and then recreate the connection afresh. If you want to try that, instructions for doing so are here:
http://www.virgin.net/helpme/dialup/connection /settingupinternet/index.html
Chris