Like Tech Analysis said... and if this machine is old enough to have an AGP video card slot... they are very sensitive to being fully seated. Even if not AGP, good idea to unseat and reseat video card and everything else except the PSU and the CPU. Should do the easiest things first in troubleshooting.
Take the box outside before opening.... And bring a can or two of compressed air while your're at it. The amount of dust inside will be amazing. Save yourself the clean-up hassle, and take the box outside first... Then go apeshit with the compressed air... Regarding the slow start-up, if it's accompanied with a lot of disk activity/crunching, you may have a dying drive or it's full or it's highly fragmented and in need of defragging. Back up the important stuff and then defrag the drive. Don't defrag and then back up! Defragging might kill the dying drive for good! Try the free Auslogics defragger instead of the native XP program - easy to find on Google. If defragging doesn't help, look into spending the $75 for a new 7200rpm hard drive or $200 for an SSD drive... Might as well as upgrade XP while you're at it unless it presents license issues with your existing software...
Simple first. Did you try system restore back to a working date? To clean it up use free CC. http://www.piriform.com/ccleaner Then from the same company use defraggler to defrag.
Lots of great advice. Much of it is really simple, and I should've thought of it. Oh well. But I really appreciate it. I haven't touched it yet, due to being busy having a fun weekend. I'll work on it later tonight and let you know how it goes. It might just be old and dying, but that's the way it is with an old machine. Thanks again