Slow Explorer init on reboot

For a long time, and also after re-installing XP Pro, when I reboot, Explorer is not alloing access to the Quick Laucnch, Task and SYStray for a 15 minute or so period after Windows has displayed the Desktop. I get an hourglass cursor whenever I try to access the tray. I have rebooted without any start-up programs being inited, and the behaviour still persists.
Any ideas?