Can I fit road tyres on a cyclo cross bike? I've been considering purchasing a Boardman Pro CX Cyclocross Road Bike Large 2010. (Broken link replaced with Google search)

The spec says the wheels are 700C and that it comes with 700 x 35C tyres. I'm wondering, should it be also possible to fit some thinner road tyres with sizes 700x20c, 700x23c or 700x25c?
Yes, you can! It looks like that bike comes with Mavic CXP23 rims, which can fit down to 19mm tires I think. You would defiantly have no problem with 700x23 tires. I'm surprised it comes with 35, that seems a bit wide for those rims, but you should be OK.