Windows XP slow startup times are a direct result of the number of applications being initialized and the quality of these programs. Troubleshooting these issues requires the use of the system configuration tool provided with the operating system.
You can find the Microsoft System Configuration tool under start menu/run/enter msconfig. It includes a tab that provides a list of all initialized applications and services as the system boots up. Each one commit disabled using the check-boxes on this tab. You just need to be careful that you do not disable applications that are critical or that you are unfamiliar with. Do a quick search online to see what these executable files due before making a choice to disable them. They can of course be enabled again at a later stage if you so wish.
For those applications that you leave enabled, you then need to check how much RAM their using. This can be identified on the task manager processes tab. Any of those software utilities applications which appear to be using an excessive amount of memory should be checked for updates. Many manufacturers of services/applications provide performance packs, fixes and updated DLLs and device drivers which will help boost performance of Windows XP slow startup times.