OK, with the help of an internet document, I figured it out on my own. Some luck was involved though.
I found out that if I boot the computer in safe mode, I can then log on as administrator. My own default ID had full administrator privileges, but since I did not have the password, I needed to log on as the system administrator. Apparently in XP that can only be done in safe mode.
Once there, I still needed to know the system administrator password. I tried the two obvious defaults... a blank, and then "admin". Neither worked, suggesting at some time in the distant past I had also put in my own admin password as well as one for my default account. I tried guessing at favorite passwords and lo and behold, one of them hit. Once I got in as system admin, I was able to then reset my default account password.
The lesson I have learned from this is that if you are going to have system passwords in XP, be sure to create a password backup disk. The help menu in XP explains how to do this. I never knew before today such a thing existed.
There are third party products out there for this problem, but some are unreliable, and some cost money. And there is always the problem of not knowing enough about who is providing them.
Until I began RVing, this was never a problem as I did not use boot passwords. But now I do because of the decreased RVing lifestyle security.
A serious hacker can have their way with the XP password security. But I found out today it is a pretty serious system against ordinary pokers like me. ;D