Intermitent boot hangup

I have a fairly new computer that has a full time virus and spyware scanner. When I start it up, about 50% of the time, it hangs up for about 4-5 minutes at the black Windows XP screen; the one prior to user logon. The tape below the Windows XP logo intermitently travels across, then hangs. Any ideas what is loading at this stage of the boot process and how could I attempt to fix this?