<p>Sometimes my Oracle database on Windows gets hosed. How do I do a manual uninstall of Oracle?</p>

Assuming a unix type OS and that you properly installed it using an account named oracle...
find / -user oracle -exec rm -fr {} \;
That having been said, this must be done as root and you had better not mind loss of any and all files that belong to oracle.  There will be no... NO recovery from this method.