If you haven't added anything new, the cache files are most likely the culprit.
I always delete them before or after I play my full CC collection, before and after using Bodyshop, and when I switch between full game and near to CC-free. Less chance of bugginess. I've not had startup problems after I started doing that.
This empty Groups cache file might save you having to remember to delete said file every time. It is posted on an open part of the Forum, so no need to register.
I am starting to see this also, with Win 7 64bit Home Premium, while playing last night the game was chugging at 7 frames per second. I am running a Nvidia 8800GTS 512Mb and 4Gb of RAM.