Hi and welcome to the forums.
I assume you live in the UK like me so I think I can help.
I use 3 mobile broadband which in my opinion is the best of the PAYG tariffs. You can buy a dongle which is the expensive bit or if you have an unlocked 3G phone which allows tethering you can use that if you know how to set it up.
You get 150MB of free data when you top up (minimum £5 in store, £10 online) plus it also works as a phone if you want it to.
This 150MB of data lasts 90 days or until you use it all up.
This means it works out you spend £1.66 for 50MB of data every month, which if you have static broadband at your home should be enough. If you ever use more than this just top up more regularly.
For more info (and probably a much clearer explanation visit
3 - Pay As You Go - SIM Only - Free stuff - Free stuff with every top-up)
I don't know too much about Virgin medias mobile broadband but a good place to start would be here
Mobile Broadband - Broadband - Virgin Media
Hope this helps, Oli.
EDIT: After reading your post more carefully, it sound like your do not have your Internet connection going into a Wireless router and then to your laptop and desktop.
This suggests to me that your Virgin Contract may be quite old as all new tariffs include a free Wireless router and setup, not to mention less to pay per month (I went down this road with BT, tripled my connection speed, got a load of free stuff and I now pay £10 less per month.)
Have a look at
http://allyours.virginmedia.com/websales/product.do?id=3264, if your contract is quite old there is a contact us box with a few phone numbers in, It might be worth a call, you never know

.
Oli