Have you tried to use the modem with an ethernet cable, rather than wirelessly? It's the first thing you should try; that will tell you whether the problem is with the modem (router?) connecting to the cable service or with the connection between that device and your computer. (Most routers are supplied with an ethernet cable. If not Poundland will sell you one for the rather obvious price of £1).
If you still get a really slow speed with an ethernet cable then there's either a fault with the modem/router or with the actual cable to your house. (In either case you'd need to contact Virgin Media). If you get a good speed with an ethernet cable but still get a ridiculously low speed with a wireless connection, you might need to change the router's transmitting channel. (The instructions should be in the manual which, hopefully, came with the device. Otherwise post again and we'll try to help you).