Don't trust a file? Use SandBoxie.

Sandboxie runs your programs in an isolated space which prevents them from making permanent changes to other programs and data in your computer.
Works very well. I have used it for over 2 years and i have never had any real virus problems.

I highly suggest getting it. It can be downloaded here:
http://www.sandboxie.com/index.php?DownloadSandboxie

I never even have to worry about viruses with Sandboxie, AVG and a few other PC tools.

Hope this helps.

Thank you