Uninstal ORACLE DB

I have faced with some failure situation that "Checking for Oracle XE service instance

Safe way:
goto oracle UniversalInstaller and uninstall oracle properly. if you don’t find it in your system, just click on the same setup file and remove.

another:
Click Start, type ‘cmd’ in the search field, and when ‘cmd’ shows up in the list of options, right click it and select ‘Run as Administrator’.

C:> sc stop OracleServiceXE                        (to Stop service)
C:> sc delete OracleServiceXE                      (to remove)
C:>sc delete OracleServiceXE

if still you have problem you may not solve but we have to try ...

http://www.oracle.com/technetwork/database/winsoft-098398.html

On Windows the ORACLE_HOME may also be stored in the registry:

• HKEY_LOCAL_MACHINE\SOFTWARE\ORACLE
• HKEY_LOCAL_MACHINE\SOFTWARE\ORACLE\HOMEID
• HKEY_LOCAL_MACHINE\SOFTWARE\ORACLE\ALL_HOMES
• HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services

Remove any oracle entries in the PATH variable under Computer =>Properties => Advanced System Settings => Environment Variaiable => PATH
In the same place remove the ORACLE_HOME entry

START -> RUN -> Services.msc
Locate all Ora* services & click on STOP
Start -> RUN -> Regedit
Locate HKEY_LOCAL_MACHINE folder in the registry
Open Software folder
Delete the Oracle folder under Software
Open System folder in HKEY_LOCAL_MACHINE
Open CurrentControlSet folder within the System folder
Open Services
Delete all keys related to Oracle. Every key starts with “ORA..”
Close the regedit
Delete ORACLE_HOME folder
Delete the Oracle folder in Program Files
Reboot