I've been running Vista HP for 2 years, this month. It's fully loaded with every program that I need, some music, a few videos I produced for YouTube, documents that I need for work, a backup or two of my web site(s), etc, etc, etc - and my Vista install is consuming 28 GB on my HD, sooo...
Yes, for me, 40 GB would be plenty!
EDIT: I might also mention...
I have a clean install of Windows 7, e.g. nothing but the core system and some AV software (to get rid of the nag) and it uses 18 GB.
I never ran a clean install of Vista. I bought Vista pre-loaded on a laptop computer, which was loaded with cruft (took me 2 days to get rid of it all), however, reportedly a clean install of Vista takes up 15 GB of space, which is consistent with my W7 experience.
EDIT 2: Here's a shocker for you...
I run Linux Mint 6 x64 on the same machine as Vista HP (28 GB) and Windows 7 (18 GB), and Linux Mint 6 x64 is only using 4.5 GB of space.
I installed VectorLinux 6 Standard Gold on an ancient Fujitsu LifeBook yesterday (10 GB HD, 550 MHz Celeron-M, 64 MB RAM). It works fine, albeit slow, and I still have 6 GB of space left. I'm thinking about dual booting it!
Sooo... if you're hurting for resources, you might give Linux a shot!