China DOS Union

Does overwrite installation have to be done under DOS? (I heard it can also be done under Windows)
Will overwrite installation overwrite the registry? (I heard you don't need to reinstall software that's already installed)
What's the difference between overwrite installation and reinstalling?

Overwrite installation can be done in WINDOWS. I've tried several times upgrading WIN98 to WINME that way. Of course it can also be installed over WIN98.
Overwrite installation will generate a new registry; you could also say it overwrites the original registry.
Compared with reinstalling, overwrite installation doesn't require reinstalling other software, drivers, etc. It's suitable when the WINDOWS system has minor damage and you don't want to reinstall everything to save time. But an overwrite reinstall will leave behind quite a bit of junk, and the original overall hidden dangers may still remain.
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A correction: the "partial hidden dangers" above should be "partial hidden dangers"

After a new registry is generated, many programs will have problems like "dll errors" and can't be used. Is that what you mean by junk, these malfunctioning programs?
If so, then how should I understand the sentence "there's no need to reinstall other software"?

I've always had an idea: when the machine is working normally, export the registry as a backup (A) and keep it permanently. Before installing each piece of software, first import this backup (A) to make sure the registry is intact; after installing the software, immediately make another backup (B). If it works fine after testing, then just keep backup (B)... and so on. This way, when a system problem appears, just do an overwrite installation and get a perfect system; then import the latest backup and get a perfect registry; then match it with those programs that have lost their heads, and it'll just happen to put their original heads back on!!!
But hearing what you said, about "junk" and "the original overall hidden dangers," it seems I was thinking too naively!!!
Then I really do need to ask clearly what this "junk" and these "original overall hidden dangers" are all about. And why is it that with what I mentioned above, you still don't need to reinstall other software (especially those that have lost their heads)?
ps: head -- refers to the part of a program that's written into the registry

The WINDOWS operating system is a relatively complex operating system. For general system problems, besides registry problems, there are also many cases of missing or incorrect system files. This mainly refers to when you incorrectly install and uninstall some application software, which may overwrite or delete shared system files or DLL dynamic link libraries (mainly under C：\WINDOWS\SYSTEM or SYSTEM32). Reinstalling the operating system (what you call overwrite installation, or upgrade installation) can restore these system files, but it does not overwrite the registry; it partially repairs the registry (that is, repairs the system-related part). So installed application software doesn't need to be installed again, but the junk in the old registry still remains (mainly the application-program part). At this point the system is still not a perfect system. Only reformatting and doing a clean installation is the cleanest. Of course, doing a full new WIN installation is already the last resort.
Your method really is a pretty good operating habit, but most application installs also copy some DLL files or OCX and other system files into
C：\WINDOWS\SYSTEM or SYSTEM32, and uninstalling means deleting all files in the reverse order of the original installation. (Sometimes it will delete some shared system files.) If some incorrect installation or uninstallation is done, some system files will be left behind or deleted (a DLL error means some DLL file exists but is not registered in the registry, or a DLL file that is registered in the registry has been deleted). Therefore, simply restoring the registry alone may not solve the problem.
So the correct method is to install and uninstall properly. For some shared DLL files, if you are not sure whether they are being used by other software, you'd better choose not to delete them (there is usually a prompt during uninstall). There are some tools that can monitor the entire installation process of application software. (Including which files are copied and the stuff written into the registry), and generate a record. You can use that to uninstall programs cleanly and thoroughly.
Let me remind you: if you have a habit of trying out software, then you should always be prepared to reinstall the system. No expert can avoid this. Hehe!

My own method for everyone's reference:
After installing WINDOWS, IE, OFFICE, hardware drivers, and some commonly used programs that aren't upgraded often, point the target folders of "My Documents" and "Favorites" to folders on drive D：, then use GHOST to make a backup of the system on drive C.
Although this kind of backup isn't "perfect," and after each restore you may still need to install some software, this system has relatively little junk, and you can also test-install all kinds of software on it without worries. Once the system goes bad, it can be restored in a little over 10 minutes.

I just don't know how to use GHOST: asking for advice

There's a detailed explanation in the beginner section..

Using GHOST is much faster than restoring the registry, and much more reliable too.