shut down database . listener . every thing about oracle .. shut it down .

run the universal uninstaller and de-install product . ( by removing oracle home) .
after it finish , go to properties of MY COMPUTER . and remove oracle home and oracle sid from ENVIRONMENTAL VARIABLES.

after that run REGEDIT. remove oracle entries . except for Microsoft driver for oracle.
Uninstalling Oracle 10g Manually from Windows XP Krishnamoorthy’s Blog

restart and delete the folder in i.e . c:\program files\oracle
and the oracle home folder where you have installed it .


OR GOOOOGLE IT .