Sounds like its time for a reformat. There are a lot of things that cause startup to slow down, but the biggest thing is usually that xp gets bloated over time. Doing a reformat is the best way to fix this.
There are a lot of little things you can do to speed up the boot time, but if its taking 3-4 minutes, I don't think much besides a reformat will be able to fix it. A few things that affect the boot time. You can turn off floppy seek in the bios. The ati control panel is supposed to add about 10 seconds to boot time. You can also tweak some bios options. Download a progam called "ccleaner", it will clean a lot of crap from your system, and the program is very clean and doesn't carry any spyware. I believe toms did an article on how to configure all the little bios options to speed stuff up. Realistically, all those little things will take a 30 second boot time down to 20, but anything like 4 minutes I would recommend reformatting. I usually end up doing it about every 6 months.