I should note, that if you are in the middle of an install, and you get stuck at this point, but really do not want to have to restart the install again ... then go to a command line (Ctrl-Alt-F2) and find the migration assistant
ubuntu@ubuntu $ ps -ef | grep migr
root xxxx xxxx x Nov19 ? 00:00:00 [migration/0]
root xxxx xxxx x Nov19 ? 00:00:00 [migration/1]
root xxxB xxxA x Nov19 ? 00:00:00 log-output -t ubiquity --pass-stdout /usr/lib/ubiquity/migration-assistant/ma-apply /usr/lib/ubiquity/migration-assistant
root xxxC xxxB x Nov19 ? 00:00:00 /bin/sh /usr/lib/ubiquity/migration-assistant/ma-apply /usr/lib/ubiquity/migration-assistant
root xxxx xxxx x Nov19 ? 00:04:11 /sbin/mount.ntfs /dev/sda1 /mnt/migrationassistant -o rw,umask=0022,nls=utf8
root xxxD xxxC x Nov19 ? 00:00:00 log-output -t migration-assistant /usr/lib/ubiquity/migration-assistant/ma-import --target=mymusic --ostype=windowsxp --fromuser=gary --frompath=/mnt/migrationassistant --touser=gary --topath=/target
root xxxF xxxD x Nov19 ? 00:03:43 /usr/lib/ubiquity/migration-assistant/ma-import --target=mymusic --ostype=windowsxp --fromuser=gary --frompath=/mnt/migrationassistant --touser=gary --topath=/target
root xxxx xxxx x Nov19 tty2 00:00:00 grep migr
ubuntu@ubuntu $ sudo kill xxxF
Note, I changed all of the actual process IDs, but I otherwise hand-typed everything else. The first set of xxxx numbers is the actual processID. The second set is the ID of the parent process. You'll note that the sequence goes from the earliest command, and each command spawns the other until the final migration-assistant process (the one that will be accumulating CPU time). That PID (xxxF in my example) is the one you will want to kill.
Note2: Process IDs do not ever have x or F. I used a non-numeric in my example to make harmless those who would actually type sudo kill xxxF, assuming that the process ID must be the same, instead of actually going and finding the number.
After that, the installation finishes, but your stuff will not have finished importing. Go check your home directory manually, and you may be deleting or re-copying things yourself.