first off if by upgrade you mean actually upgraded with by the installer doing everything automagically, then i'd say you should do a fresh ("custom") install. it clears away all the cruft you're bound to have accumalated and stops it being dragged into you new and shiny system. but that's just me
second, iirc, after you've downloaded and installed a program with impulse it only keeps the files required for your OS, be it XP/Vista/7.
i found that out after trying to transfer and archive from XP to a vista machine to save on bandwidth. a way round that issue is select 'archive and install' so you have an archive with all the required parts. porbably move it from its default location too to stop it being over written.
with that i surmise that during upgrade MS copied all the game files across but you haven't got the right files for the win7 OS.
you could try 'verify install' (right click on the icon) to see if that gets things sorted.
or just uninstall and redownload (but you will loose any single player data, unless SD has a neat trick for that??).
ftr, i'm running happy with win7 64-bit and the custom install took me an hour, including time to copy my XP files and what-not to another disk. was playing DG an hour after that (gotta love the download speeds!!)