Preventing Windows XP system steadily taking longer to start-up

Another thing to try is remove all unnecessary startup programs.  If you take a look at the "Notification Area" (what was previously called the System Tray) in your Taskbar, you will probably see a number of items.  See if you can find an option for removing those.  Also type MSCONFIG in the Run dialog box off the Start Menu and click OK, and inspect all the  programs that are on the Startup tab, to see if any can be disabled there.  Finally, there is the free Microsoft Bootvis tool that can be used to analyze startup time to pinpoint what the hangup is:

http://www.softpedia.com/progDownload/BootVis-Download-3465.html

BootVis is a performance trace visualization tool for use with Windows XP systems. Customer research shows a frequently requested feature that users want from their PCs is fast system startup, whether from cold boot or when resuming from standby or hibernation.
 

Unfortunately, the best way to fully get back to faster boot speeds is to backup data, reinstall windows fresh, reinstall programs, then restore the data from backup. Windows just tends to get bloated  over time from programs being installed, uninstalled, reinstalled, drivers being updated, you name it. Bits and pieces of things get left in the registry, abandoned files build up and so on.

Without the full reinstall however, there are some things that help.

Uninstall any programs you no longer use, especially any that have any components that start up automatically.
Run msconfig, and look through all starting programs and disable any that don't need to have started automatically.
Disable services that aren't needed. A great guide for this can be found at http://www.blackviper.com/WinXP/servicecfg.htm
Remove any unneeded fonts in the fonts folder
You can clean out the registry using registry cleaning tools. I personally don't use them, as some have been known to cause problems. I prefer cleaning the registry myself, however that also shouldn't be done without knowing for sure what you are doing.
Run spyware checking programs on a regular basis and anti-virus.
There are utilities out there that can search your drive for abandoned files (I haven't used any for awhile but used to use Cleansweep a long time ago, I just tend to be a purist these days and do it all manually or just reformat and reinstall. Hopefully someone else here might have a good recommendation on a program to use for this).
If you've had hardware installed before that you've since removed and no longer use...go to a command prompt and type:
SET DEVMGR_SHOW_NONPRESENT_DEVICES=1
START DEVMGMT.MSC
Then in device manager go to View|Show Hidden Devices
Look through all devices and delete those that relate to your removed hardware. (They should show up as slightly greyed out).

Go to Start|Settings|Printer and Faxes
Then hit File|Server Properties
On the Drivers tab, remove any printer drivers that you no longer use.

Heh, just a few things to look at :)