You can ensure you have an unaltered Bitcoin by checking it's signature. However this is more burdensome than checking it's file hash, which is below, from my copy of the installer retrieved on Sep 13 2013
>md5sum bitcoin-0.8.5-win32-setup.exe
6cff750efbae30d14f97f663d18aacf8 *bitcoin-0.8.5-win32-setup.exe
>sha256sum bitcoin-0.8.5-win32-setup.exe
6f6b8fd68f56a8e700090267c53aa592b9c9e5c993f44c7be11ba9b87e1f92bb *bitcoin-0.8.5-win32-setup.exe
>fciv -both -add bitcoin-0.8.5-win32-setup.exe
//
// File Checksum Integrity Verifier version 2.05.
//
MD5 SHA-1
-------------------------------------------------------------------------
6cff750efbae30d14f97f663d18aacf8 c6ecb5c1447c57fc0be4c69c4f300fb9fb41adf0 bitcoin-0.8.5-win32-setup.exeThe last tool is from Microsoft:
http://www.microsoft.com/en-us/download/confirmation.aspx?id=11533md5sum or sha256sum for windows, I'll let you use Google to find your own copy.