Playing with registry might cost u very much.......
Be sure wat u r deleting in registry.....

Before u delete try checking this in registry.....

HKEY_LOCAL_MACHINE\SOFTWARE\YOUR_ORACLE_BASE(DIR)\HOME

ckeck if u still have the ORACLE_SID set as ur DB name in this path.

also remove from environmental variables if Oracle_SID was set......

check if the OracleServiceYOUR_DB_Name still exists in the services.

regards
Abhay.