Disk A

[s1mple4es]

I have the OC on disk A, and the *.msi file installs to disk C, which I don't have. How can I fix this?

[undefinedcode0]

in disk management, select your a: disk, change the letter and follow through to change it to c:

[s1mple4es]

how do I change it?
when I open the file, it immediately displays an error with the disk

[s1mple4es]

in the method you described, you'll have to change drive A to drive C, which will cause many applications to crash

[undefinedcode0]

thats for any applications that have hardcoded paths, if any do. i suggest you reinstall them.