Disable the Win7 auto updates? Set up a system restore point and save it once you have a setup that works? Disable all auto-updating background services?
Serious Advice to anybody:
I DO recommend the tool DiskSpace.
It scans your HDD or any partition and tells you exactly which folder is how big, how many files in it, and you can step down and up in the folder hierarchy – giving you a complete insight on where the hell your space went.

(coloured areas may be clicked, and Disk Space will again open the selected folder into a more detailed diagram).
No, this ain’t a screenshot from my computer 😀
Superuseful! 😉
(but I don’t know whether it exists for MAC)