Windows Activation

[Amy]

When restarting my computer, at the Windows XP Startup
screen, I am prompted that I need to activate this
version of Windows before logging on. After clicking yes,
instead of being taken to an activation screen, it
returns me to the logon screen. I am unable to gain
access to my desktop.
Have tried reseting system date in order to activate from
an earlier date, but have had no luck.
Any ideas???

Thanks!
Amy