If you have been using Outlook Express then you will find the information under Tools - Accounts - Mail. Otherwise you will almost certainly find it on the ISP website and on literature supplied by Virgin. For most ISPs the incoming server is something like pop.ispname.com, and the outgoing server smtp.isp.com but there are many variations
There are no overwhelming advantages to using either Thunderbird or Outlook. I would recommend that you initially use Outlook Express, then if you want to you can install Thunderbird and use it's import function to import the settings from OE. In my opinion OE has one big advantage over Thunderbird. You can make a backup copy of the mail files (Inbox.dbx, Outbox.dbx etc), and read them using DBXtract without having to import them into OE. Thunderbird mails can be backed up using Mozbackup, but they have to be imported back into Thunderbird to read them.