Create a new blank folder on the root of the C drive and call it PostgreSQL. If you have a 32 bit machine, download
this file and if you have a 64 bit machine download
this file. Run the installer you just downloaded and have it install to that new directory (C:\PostgreSQL\) and that will be more likely to work.
We use the default password dbpass for the PostgreSQL user (when you get to that stage in the isntaller) so I'd recommend using that.