I think I may have some corrupted system applications, and when I tried reinstalling XP, it didn't overwrite the existing files, but instead just checked to see if they were there by comparing names.
I can turn on the computer, and if I don't use the Windows Explorer, the system actually runs quite a long time. But there must be another file that also crashes the system... either Windows Explorer uses that file, or that other file uses Explorer, I'm not sure which.
Also, there was nothing wrong with my router. The connection to the internet changed IPs, so I just needed to set the router to obtain the new IP automatically, instead of using a set configuration. I'm going to go check my laptop computer and see if it will now connect to the internet.