These things DO fail after awhile. The fact that it is failing to be recognized by multiple systems suggests that it's dead, and the data on it is gone.
I decided to re-install XP on my Dell but first I saved almost 2 Gbytes worth of data onto a memory stick.
After the re-installation of XP I find that the USB stick cannot be read. In fact, when I try the memory stick on other computers I have the same problem. The memory stick worked on the old installation of XP after data had been saved to it.