Hi. I work in a CAB and we have taken on a couple of IT volunteers this year. Each have significantly improved the way we work and suggested using some of IT more efficiently so its been great for us. The big problems has been that both have gained some real world experience and a reference and have quickly gone on to find paid work. No problem with this as its part of what we do but once we realise how useful having someone with decent IT skills can be it makes us want someone long term. This effectively means trying to find a retired IT person who can work with an office over a few years. I know many larger CAB have someone like this or even employ IT staff to maintain, upgrade and develop their systems.
You don't say where you are but Im in a small Dorset CAB and if your anywhere near us I'll be happy to discuss this further.
You should also try your local Council for Voluntary Service as they may know of a charity looking for someone to help in your area. Good luck