You say you are running XP Pro.. a few additional questions for you:
Are you running in a domain environment? Are the users you are loading local or domain users? If you use a domain, do you have any GPOs applied to the machine? Or any firewalls present on the network? Especially that may prevent communication with the PDC?
Also, to see what you run in startup from MSConfig or Hijack this would be useful. Some startup programs (esp. drivers or virus scanners) insist on trying to load before the user interface. Identifying, reinstalling or removing those can quite often help. Bloatware like adware/spyware can indeed increase this time.
When did this behavior start? Have you looked at system restore to see what sort of rollback points you have around this time, and if there were comments appended to that?
Is this behavior replicated when you use THE Administrator account? (not *an *administrator, but the system account) Does this behavior also exist in safe mode, or is it better?
Just a few questions that may lead the way to an answer...