Can I use my old Virgin Mobile SIM on a Motorola W377?
Is this the phone you are talking about? http://www.gsmarena.com/motorola_w377-2153.php
If it is, then it's a 2007 model, how old is your SIM card? It's 1 or 2 years newer or older, it should work without problems.
Did you try inserting sim and see whether it works, if not, may be you will have to unlock it.
from which country you are ?
In the US, Virgin Mobile uses a different technology than SIM. It shouldn't even have a SIM card.