It shouldn't be a problem to connect from 8i to 10g as long as you don't try to use any 10g SQL*Plus features (e.g. purge recyclebin). Shouldn't be a problem to install multiple clients on the same Windows machine either - just be aware of which one you're running your executables from to make sure you're accessing the correct tns config files, or set your TNS_ADMIN variable to override the defaults and point to a centralized location. If you have trouble, just search metalink for "oracle.key" and/or "tns_admin windows" and you should find some notes to get you on the right track. This probably goes without saying, but just in case you're not aware, you'll want to make sure you install the new client in separate directory from the existing client. Also, why install the 9i client instead of the 10g client?
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