If your having security certificate problems, an upgrade to internet explorer 8 (i would stay clear of 7) or even better get your self a copy of Google Chrome (much better) http://www.google.co.uk/chrome and see if you can now get online.
I would guess that the trusted security certificate you have for virgin has expired and needs renewing. Virgin should be able to help you with this. Would also be worth checking the time / date on your pc, if they are wildly out, this can stop certs from working.
Would be surprised if having 3 pc's on your broadband connection would be a problem, i have used Virgin in the past and had up to 6 connected with no problem. It would be worth giving them a call though.
Is it the Flight Sim or the computer that needs the date and time set each time it is started?
If it is the PC then the chances are that it needs a new BIOS battery for the motherboard. Depending on the motherboard this can be a simple or complex change. Some of the old motherboards actually had the battery soldered to the board, but most later ones tend to use an easily replacable button cell. Something like Belarc (a free program) http://www.belarc.com/free_download.html will identify all the relevant parts of your system and allow easier tracking down of any required replacement. On the other hand if you actually got the motherboard manual with the computer this should give all the details and show the board layout.