Was the problem there before you fitted the mobo'?
If not did you reinstall XP after. You may need to do this as XP will be set up for your old mobo' and could need differant drivers for your new one. also, were the drivers that came with the new mobo' installed.
Go into control panel and click on 'add new hardware'. If it comes up with a list of new hardware then you'll need to install these and the new drivers.
If this doesn't work backup all your data and do a fresh install of XP.
If after this you still have problems make sure that all your add on cards and ram are fitted properly.
As a last resort suspect a faulty mobo'.