If you've not already got a digibox (or a TV with built-in Freeview) you'll need to get one soon as all analogue transmitters will be switched off by 2012 at the latest. (Some areas are already 'digital only'):
http://www.digitaluk.co.uk/when_do_i_switch
However all TV broadcasts (both analogue and digital) are transmitted on the basis that viewers should be able to get decent reception if they use a good quality OUTDOOR aerial. Some people are able to get good reception using an indoor aerial but that has never been the intention of broadcasters.
At present, in those areas where the switch to digital hasn't yet been completed, the digital transmitters are currently operating on REDUCED POWER (in order not to cause interference to analogue transmissions). So it's unlikely that you'd be able to get a digibox to work until the analogue transmitters have been turned off (and the digital transmitters switched to full power). Even then it's unlikely that your present aerial would be adequate. (If you can't get decent analogue reception it's unlikely that you'll get any Freeview reception at all).
You may well have to install an outdoor aerial. However it could be worth trying an indoor aerial which has a built-in signal amplifier. Some of those on offer only give a minimal increase in signal strength but this one provides excellent 'gain':
http://www.maplin.co.....aspx?ModuleNo=115127
Chris