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I have extracted the files and run the psotgresql-8.3.msi and after the language selection I get the message (since i get in Swedish I have to translate it) "WINDOWS INSTALLER The installationspackage can't be opened. Confirm that the package exists and that you can reach it, or contact the program seller to confirm that it's a valid package for Windows Installer."

I have disabled UAC.

Try re-downloading it, maybe using another browser.
You could also try running it as administrator (having UAC off is good, but you may still need to do this).

Alright i tried downloading it using explorer and tried downloading an other verison but still the same problem.
When I right klick the msi files I dont get the option to run as administrator.

Try installing 8.3.8 - 8.3.8 is newer than 8.3.5 and contains several security and crash related fixes, and it's completely compatible with 8.3.5 databases if that's your concern.

Tried that and still the same problem

Have a look at this: How to reinstall PostgreSQL - that walks you through the whole process.