Very Slow Startup after installing Windows XP Service Pack 2?

I recently changed back from Windows Vista back to Xp Professional. I did a full format on my hard drive and did a fresh Windows install. I did not have any problems until I installed service pack 2. Now when I turn the computer on, I get a black loading screen before the windows welcome logo. It is a solid black screen with a white loading bar on the bottom. It takes the computer roughly 5-10 minutes to load before I get the Windows XP loading screen. Everything after this black and white loading screen runs fine with no slow downs. I have tried running Registry Mechanic, and Spyware and Virus detection software and have found no problems. I have also installed the Windows Service Pack 3 and all available updates, and I still have the problem with the Black loading screen. Does anyone know of any solution to this problem, or know if anyone else is having the same problem?