Four easy steps to delete an old Windows Vista installation:

1) open admin command prompt

- Click start
- Type: cmd
- right-click it under programs
- click Run As Administrator

2) take ownership of everything (will take a while)

In the command prompt:

cd windows
takeown /F . /A /R /D Y > NUL

3) grant yourself permission to everything

In the command prompt:
icacls . /grant:r Administrators:(F) /T /L /Q

4) Delete everything