It will "run" at that size but it won't be useable.
Just try running it with 512 and it will go virtual and drive your performance down to a crawl. A brand new Vista Home system with no apps running typically will demand 860MB of RAM If you only have 512 it will constantly swap to disk and that destroys performance. 2 GB is the practical configuration. Microsoft claimed XP was fine in 128 but you could never do anything with it with that little RAM. An extra gig for that computer is like 50 bucks. Money very well spent.
You'll have to forgive me. I have only been in this business for 24 years and I make a living as a IT consultant.
Paul