Aljaz,
Not trying to go off tangents follow these steps meticulously,

1. In configuration box(my computer) /services, you must have oraclestart and oraclestart start , if not start them.

2. start your TNSLSNR80 from services or thru cmd.exe prompt lsnrctl8x , then check status first ,if not started promt start.

3. Test your connection to the database , thry net8 utility.
create it if you didn't create it. Depending on how you created the instance(i prefer to use dbassistent , then custom creation helps you create you internal account...), but anyways keep all your passwords and test the connection here...
modify your lsnr.ora, tnsnames.ora files if required.

4. Tip:also check the registry, i prefer to have it clean from previous installations...

5. Let us know here if this worked.

Cheers!!!!!

Tarun