70GB of space is used already of his 300 available.
After some tricks I discovered that 42 gigs are used by... System Restore!
I started looking for a setting to lower it's hunger but couldn't find one. At the same time I tried to explain to my brother why he needs it turned on and that this feature helped me couple of times in my life.
At some point I gave up on both. And before he'd force me turn it off I had to find some solution. I googled and found that unlike in XP you can't change amount of space used for System Restore and that by default it allocates 15% of the disk drive’s size or 30% of available free space on the drive, whichever is smaller. This is fine in my case - I have only 13 gigs free on my laptop out of 120, and SR doesn't use anything, but for him it wouldn't be a reason to leave it on.
For this I've found that you can use next command line (Start -> type cmd -> hit Enter) which will lower maximum used space to amount you specify:
vssadmin Resize ShadowStorage /On=[drive]: /For=[drive]: /Maxsize=[size]
So if you want to set it to 4GB on disk C: you would type something like:
vssadmin Resize ShadowStorage /On=C: /For=C: /MaxSize=4GB
I just want to add that Vista doesn't suck that much anymore as it did two years ago. Esspecially if you use only it's basic features like moving files around, creating and renaming files and folders and so on. I don't use any other features - I have applications for everything - iTunes, Dreamweaver, Opera, Picasa and so on.
I don't see any reason to upgrade from existing XP to Vista, but if you get a new computer there's no reason to degrade it to XP - Vista is better and more user friendly with things like better file preview, better address toolbar and others.