If it's an OEM version, and you do a complete re install, which you will have to. It will probably activate OK, but if not, a call to Microsoft will satisfy them, as you are using the same motherboard.
under Microsoft eula it not allowed. If you have the original CD then go for activation, if it refuses which is unlikely then call the freephone number which will eventually show onscreen when a new key is required, just tell them you had to clean install onto a new HDD. If your honest with microsoft they will assist no problems.
"I'm going to fit a bigger hard drive which will require a fresh XP install."
Fitting a larger drive doesn't require a fresh install of XP. You may choose to do one, but you can just as easily clone your current installation to the new drive. This may not even ask for activation.