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.
Finally, your overall PC performance should be a concern. Windows XP slow startup times can also be improved by improving disk access times using the Disk Defragmenter tool. You should also try to increase the amount of virtual memory you have allocated (via the performance options). Run Windows update service to get the latest patches and device drivers for the operating system. It is also worth running an anti-virus scan to check to see if there are any viruses which may have caused the slow startup issues. Lastly, check you have left over 5% of your hard drive empty for virtual memory and the paging file so that it may expand unhindered.