Many things can contribute to your Windows XP system booting up slowly.

Most of the times this just has to do with the startup applications.
If you would like to speed up the bootup sequence, consider removing some of the startup applications that you do not need. The easiest way to remove those startup applications is through the System Configuration Utility.

• Go to START, RUN and enter MSCONFIG.
• Go to the Startup tab and deselect the application(s) that you do not want to startup at boot time.

If not you can also look at the SERVICES tab and possibly deselect the WORKSTATION option and see if that helps. IMPORTANT NOTE: If you deselect the WORKSTATION option from the SERVICE tab and reboot, a warning message window may come the very next time which you can choose not be displayed by checking "do not show this message again" option.

If beyond that you want to know more about the boot time on your system you can visit the Microsoft website on Fast Boot / Fast Resume.
Go to the Download section and you will see software called Bootvis.exe.
It is Microsoft Boot Performance trace visualization tool and has options to optimize your boot time significantly. You will have to boot a couple of times to make this happen but I believe it is worth the effort.

Here is a little taste of what Bootvis looks like and some of the steps.

Step 1: Download the tool by visiting Microsoft site.

Step 2: Here is a typical snapshot of the Bootvis.exe window. In the left pane are various check boxes listed. I have selected all of them so we can trace each of these checked activity/resources the next time the system boots.

Step 3: If you want to create a New trace at Next Boot, you can do that from the File menu option.

Step 4: TRACE. OK. You are ready to start tracing your boot time loopholes.
Select Next Boot or Next Boot + Driver Delays and you’re ready to go.
If you’re in a hurry you can immediately jump to Step 5 and simply select, Optimise the system.

After clicking on Next Boot you will see the following window pop up:

After rebooting, this is what you are going to see.
First you will see a small message stating to "please wait for bootvis to run.." and then you’ll see a small window saying that the trace file is being created. This window disappears when finished.

Step 5: OPTIMIZE. If you decide to optimise the system, you will be asked for a system reboot as shown below:

The system will reboot and the BootVis application will be executed again. After rebooting, Bootvis will show:

On some machines that I tried this tool on, it took some time before finishing. A couple of repetitions can tune the booting sequence to a more optimized level.