Log into your email on the Virgin website. Go to the Inbox and click on “Settings” in the top right corner. Then click on the “Forwarding and POP/IMAP” tab. At the bottom of the settings there are a couple of radio buttons in the “IMAP Access”, set those to ‘Enabled”.
Then click on the link below that says “Configuration Instructions” and find the instructions for configuring both Outlook (which is likely to be currently set up using POP3) and your iPhone. Ignore the bit about Virgin not supporting devices other than the couple listed. The service is actually provided by Google Mail and their site contains a lot more information about device configuration…
http://mail.google.com/support/bin/answer.py?answer=75726
Once both are correctly set up using IMAP, both devices will be able to access the email account, and changes made on one device will be reflected on the other. E.g., marking and email read on the phone will mark it read on Outlook, deleting an email in Outlook will delete it on the phone.