Their upgrade advisor is working in a very peculiar way. I have six hard discs connected to my machine and only the C: disc has less than 15 GB of free space. The rest of the discs vary between 30 and 120 GB of free space. It did not stop the upgrade advisor to "advise" me that I cannot install Vista Beta 2 due to insufficient hard disc space...
It might be right. We had once problem with 3DMark installation because no matter WHERE the user wanted to install the proggie it needed X GB temp space in C drive (the destination install drive was ignored for this purpose).