|Slow starting computers could be many things.|
Too many background programs starting with Windows -- Run msconfig and remove most programs from starting with Windows (you need to leave MS & antivirus, others- your choice).
Hard drive too full - If less than 15% free space, delete files and uninstall programs or get an external hard drive and move 'archive' files off hard drive.
Not enough memory - XP will technically run on 500MB but runs MUCH better with 1GB or more (up to 2GB, more will usually be a waste with XP).
Accumulated junk - Run Disk Clean up - check everything.
System badly fragmented - (after running disk clean up first) - run Disk Defragment (must have 15% free space on disk or nearly).
Virtual memory not set correctly, with XP look to 1000MB or more, but 1X to 1.5X actual memory is about right.
Infection - run Malwarebytes to search for problems.
Memory problems (early stage) - run Memtest86 to test.
Bad build up of registry errors and old left overs - Forget registry quick fixes save your files to external and do a factory restore or reload operating system, programs, updates, and then your files.
I tried to put them in relative order, most basic first, work your way down the list until things are better or go through all of them except the last one for a better system. The last one is a last resort but a realistic one. Post back results for more advice.
You have to be a little bit crazy to keep you from going insane.