Hi Sahil,
GOTO run and type regedit
1. Stop the service you want to remove
2. Find the registry entry:
HKEY_LOCAL_MACHINE/SYSTEM/CurrentControlSet/Services
3. Find the service name there and delete it.
Also you can run sc delete from cmd
Note: Please be careful while handling Microsoft registry as it can cause non-reverse-able damage to your system.
Then restart SAPINST and continue old installation. Let me know your Results