I think some program is trying to access it. So do a checkup in the startup list folders
C:\Documents and Settings\%USERNAME%\Start Menu\Programs\Startup
C:\Documents and Settings\All Users\Start Menu\Programs\Startup
Delete the ones unnecessary.
Also, you can type "msconfig" (without quotes) in Start->Run & have a look at all the startup programs & services which may be causing the problem.
For starters, have a look at this website for any of your startup queries:
*www.pacs-portal.co.uk/startup_content.php