Hi Paul,
By default, XP allows 12% of the hard drive space for system restore. Each
time there is a change to the system, or on a daily basis, if there is
sufficient idle time, a new restore point is created. The oldest restore
point will be automatically deleted if there is not sufficient space for it.
Perhaps, your restore folder is full or, the old restore points have been
manually deleted. If the time you wish to restore to is not in bold type on
the system restore calendar, that date is not available and there is no way
to go back to it.
HTH, JAX