Once you've checked what apps/services are loading, look to your device drivers. Old/corrupted drivers add a LOT of boot time. (These will not necessarily provide any errors to let you know they're misfiring.) If the PC is a major brand, just look up the make/model and grab everything from the driver list provided. Innocuous looking things like "chipset driver" can add a great deal of zip to your system. If you're comfortable with it, look to BIOS and other firmware updates, if applciable.
Also, the sad fact is that XP degrades over time. I highly recommend that anyone with the technical ability to do so should simply backup, wipe, and reinstall their desktop OS every 2-4 years.
Sure, it sounds painful, but this ends up taking me far less time than trying to apply enough duct tape to keep the thing hobbling along at a decent speed.