Gash
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"next time, set at least 10 Windows installations for each of the 10 sections" - On older motherboards where XP is installed, it's impossible to install more than four operating systems (XP, Linux, FreeBSD, etc.) on a single HDD.
"what settings need to be changed so that it boots from another (third, fourth) XP." - Nothing needs to be changed. After installing the second XP, a black screen with two bars will appear when the computer is turned on. You can use the up and down arrow keys to select the Windows installation. By default, the top bar is the XP that was installed last. If you install 4 Windows installations, there will be four bars. It is impossible to install more than 4.
"what settings need to be changed so that it boots from another (third, fourth) XP." - Nothing needs to be changed. After installing the second XP, a black screen with two bars will appear when the computer is turned on. You can use the up and down arrow keys to select the Windows installation. By default, the top bar is the XP that was installed last. If you install 4 Windows installations, there will be four bars. It is impossible to install more than 4.