The Holiday Stuff Packs are installed in a different manner from the other Stuff Packs. When you uninstall The Sims 2, they will be deleted along with The Sims 2 usually; whereas the other Stuff Packs are installed in their own programme folders.
The files are placed here (your language version may be named differently):
C:\Program Files\EA GAMES\The Sims 2\TSData\Res\Catalog\BinsI have the 2006 version which includes both 2005 & 2006 as well as content formerly only available by download from the exchange, so here I find:
- globalcatbin.bundle.package 46.3 MB
- H05.bundle.package 47.1 MB
- H06.bundle.package 31.2 MB
As far as I know, no other changes are made excapt maybe a bug-fix patch or updater for the base game to the latest version.
In order to regain your Holiday Stuff content, you will have to re-install the Stuff Pack after uninstalling and re-installing The Sims 2.
note: a bit of trivia, items placed in the same folder usually don't display the custom content blip * -- so you could theorhetically place custom content in this folder if you were willing to play with it and see what works and what doesn't work