Have you thought about the BootVis tool.
15 minutes sounds as though your problems may be more serious, but the output from the tool may help you diagnose problems.
Part of the Bootvis program should produce a graph of the time taken for drivers and programs to load. Can you use that to see which program or driver is loading slowly?
How does your machine boot from a powered off start? Is the delay the Shutdown or Restart procedure?
See if this threads helps (with Bootvis instructions) and a possible solution with the Background Intelligence Service
XP temporary freeze after boot up
thread779-223070
What are your boot times like if you boot into Safe Mode?
314503 - Computer Stops Responding with a Black Screen When You Start Windows
How to Troubleshoot By Using the Msconfig Utility in Windows XP (Q310560)
Try running ChkDsk to check your drive for errors. Right click your Drive icon/ Properties/ Tools/ Error Checking.
HOW TO: Verify Unsigned Device Drivers in Windows XP
Removing adware & spyware
Before Posting a Hijack log file - Best Practices
faq608-4650
See if there are any errors listed in your Event Viewer.