Presumably you have Virgin's "Cable" broadband service and connect the PC or Xbox to the supplied Cable Modem by ethernet cable?
If so, you have two options:
1) Connect a "Cable/DSL Router" by ethernet cable to the Modem and then connect both PC and Xbox to the Router.
2) This requires that the PC has *two* network adapters installed. Connect one network adapter to the Modem as at present. Connect the second network adapter with a *CROSSOVER* ethernet cable to the Xbox and then enable "Internet Connection Sharing" on the actual internet connection (modem connection), which is the first network adapter. The Xbox will then also have internet access as long as the PC is running and online.
The reason that you cannot use a simple switch for simultaneous internet access is that the ISP (Virgin) only allocates one IP address and every computer on the internet must have a unique IP address. With a "Router", it gets the single IP address and in turn allocates IP addresses to the connected computers using one of the address ranges reserved for Local Area Networks. It then "routes" the internet traffic to the relevant computer in the local network. With Internet Connection Sharing, the Host PC actually functions as a "router" and allocates a LAN IP address to the Client device (Xbox).