I've had a problem with upgrading Parallels the excellent tool for running VMs in OSX. For some reason that I could not initially spot the install would hang in the preflight check before applying all the files. I also had a similar problem trying to force an uninstall.
To cut a long story short it appears that the install/uninstall scripts try to shut down the InternetSharing and bootpd daemons.....in my case these would simply not die through the current script and I had to intervene manually - the install then ran fine.
NB - Turns out this was actually a symptom of a problem I really struggled to find. The internal script was failing to kill the processes because weirdly the 'awk' calls where causing Bus errors. I could reproduce this with the simplest print $1 style call. Turned out that this was actually a corrupt binary - no idea how this has occurred, but replacing /usr/bin/awk has fixed it all.