I use Boot Camp on my MacBook Pro to run Windows 7. Last night I thought I would try installing Visual Studio 2010 Beta 2. Sadly, the install kept failing with an "access denied" error. I was determined to figure it out. I ran Process Monitor to see what was going on and looked at some of the log files it was writing to. One of them said it found drive E: to have the most space, but failed when it attempted to write to it. Drive E: was my OS X partition. So, I fired up disk manager in Windows 7 (diskmgmt.msc) and clicked the Remove button on the Change Drive Letter dialog box. After removing the E: drive from Windows, I tried installing VS2010 again, and it worked perfectly. Here's to being persistent.
I hope this helps someone out there.