I played more than 300 hrs, until ppl told me to get the DLC's.

The game now works, sort of on medium and still do the CTD. Used to play everything on Ultra-High.

On medium it works now and again, until I get a shiny blue deer or a just a shiny blue "random item". and I know it's time to save and restart.

I play on XP, which was never a problem before all the patches.

If you're using mods, they are the likely in cuprit. Some of them are in conflict with the newly added DLC content. Cleaning the DLC should also be done using TES5Edit, that can reduce the number of CTDs.