Try setting your Homehub to a fixed channel rather than using smart channel and if your XP uses wireless b/g and not n then change the hub to just b/g rather than b/g/n.
I would agree about keeping it on b/g/n but sometimes XP won't connect to the hub when it set that way. Try b/g just to see if it does connect then you will know that is the problem and you could buy a USB wireless n adaptor to use on the XP. You can pick them up for under £10.
have you checked that the driver for your wifi card is up to date - check manufacturers website and don't reply on windows update