Thank you for your response, but still no luck: the game took a long time (30+ seconds) and managed to save and load a saved game once when I tried with a skirmish, but when I tried with a campaign mission, the game froze again on a "please wait, mission is loading" screen. When doing CTRL+Alt+Del I noticed the process was altogether unresponsive.
I think Origin is the problem, since RA2 ran fine before on this PC. I would just install and play the CD version of RA2 again and seek out a CD copy of YR, but now the installer will no longer start when I try to run it from the CD no matter what compatibility options I try, and I have no idea of why that is...