It would tenatively seem taht my problem at least lie in a scrambled Set CLASSPATH= entry in my CONFIG.SYS. Instead of neat path entries separated by semicolons, there were a lot of repeats and some nonsense entries run together.

I speculate this was caused by the uninstall/reinstalls; perhaps the parser that reads/writes CONFIG.SYS got confused somehow? In any case, for certain problems, this is a place to check, surely for DOS-based puters. Those running NT-based computers are not likely to have a CONFIG.SYS.