Just thought I'd post a short note that it appears that Dropbox amazingly hangs both VirtualBox and VMware Server installation on Windows 7.
I initially had trouble installing VirtualBox, which hung around the 80% mark (estimated based on the progress bar), after it asked me for permission to install various drivers (USB, network, etc). Coincidentally, VirtualBox released a newer version, 3.2.12 a few days later, and I gave it a try, yet it failed at the exact same point.
I gave up on VirtualBox at that point, and decided to install the discontinued (and much more system intensive) VMware Server 2.0, but to my surprise, it too hanged during installation.
After some fiddling, I gave the age old "requirement" to close all open programs I could find, and to my surprise, both installation went through without a hitch. I suppose I could have left things be at this stage, but I went on to attempt to isolate what was causing the installation issues on my system, and to my surprise, it turned out to be Dropbox.
Windows 7 (64-bit)
VirtualBox (3.2.10 and 3.2.12)
VMware (2.0.2 build 203138)
ps. There doesn't seem to be too much information on this on the web, so if this happens you as well, drop a note in the comments on the various software version combinations in your case.