This forum does not have a "I learned about computers from that ..." category, so I'll enter this post as a "Tip" in case it can save someone else from the problem I had.
Since September 1999 when I bought my current computer, it has never worked right. The biggest problem was screen/keyboard/mouse freeze-ups, i.e., with no warning and for no cause everything would just stop, usually at the most inopportune times. During Y2K I had occasion to reload Windows98SE three times for various reasons, but things never really got much better. I gradually learned to minimize the lock-ups by keeping as few programs running at one time as possible out of the 40 or so application programs I had installed.
My most recent crisis was precipitated when I installed a 10-100 ethernet card so I could sign up for DSL service. Every time I tried to load the NIC software it gave me an error message that indicated that I should reload Windows98. So I did -- and everything immediately got worse: the Backup utility would not run; the MSConfig utility would freeze-up the system; and other weird things would happen. So I reloaded Windows98 again (are we keeping count here?) with no improvement. After several calls to the DSL tech support people, I finally got DSL working and it has been reasonably stable. The failure of the various utility programs was something I just learned to lived with -- until one day I tried 'MSINFO32.EXE>Tools>Version Conflict Manager' to see if I could fix anything. That utility listed about 8 current system files that had version numbers older (lower) that the previous copies of those files. When asked if I wanted to replace the current files with newer (previous) versions. I said 'Yes'.
Ouch! That was the wrong thing to do (or, in retrospect, maybe it turned out to be the right thing because it forced me to take drastic action.)
The first symptom that something was really wrong was an unusual error message, to wit: "Spool32 - This program has performed an illegal operation and will be shut down." followed by a system freeze. After rebooting a few times, I began to get a similar and just as fatal error message: "Explorer - This program has performed an illegal operation and will be shut down. [file name] caused an invalid page fault ..." At this point 'Ctrl-Alt-Del' showed that no system programs of any kind were being loaded and Windows would not run at all. So I decided that it was time to start over from scratch, something (I admit) I had always been a little curious about doing and had not been sufficiently motivated to try. Now I was so motivated.
Fortunately DOS still worked so I was able to copy all my data onto ZIP discs along with other useful things like .ini files and other setup information before I wiped HD C: clean with FDISK and FORMAT. (I had reasonably recent backup disks written by the system Backup utility before it stopped working but I had no confidence that they would be readable given all the trouble I now had.)
Well, there's more, but I'll stop here. After two weeks of being tied to my computer I have Windows98SE reloaded and about 90% of my application programs working -- and so far no unusual or frequent system lockups. Of course I lost all my Windows updates including IE 5.1, but I am more than a little gun shy to do some of those updates again. Maybe in a month or two, or whenever Bill G. releases another operating system -- whichever comes first.
Moral: If your computer is working reasonably well and you are diddling around and come across an option affecting system files that says "Do you want to do this?" -- Don't.