I downloaded DM 4.5 and had this problem after rebooting -- all the temp files had been cleared so RMM couldn't finish installing.
Bounce1111 has the answer - worked like a charm. I did mine a little differently...
I still had the DM .exe file around so I doubleclicked on it as if I were going to install DM again. This caused the .exe file to reextract its contents in the same temporary directory as it had previously done. Note that I didn't go any further than double clicking on the .exe file. The DM setup wizard came on and told me I was getting ready to install DM, but I never clicked NEXT (the first setup screen is simply an indication that the files have been extracted). Anyway, once I saw that screen I went back to RMM finished the Roxio Media Manager installation. It prompted me to restart, at which point I cancelled out of the DM installation and restarted.
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