Modify a DLL is considered a security risk. That's how virus infects system files. As a result, Windows can identify if a DLL is from a trusted source (either installed via a digitally signed installer or cab file) and wether or not it has been tampered. In current Windows 7, the OS does not take immediate action if such tampering is detected. But I have seen in some Citrix based app server, such DLLs will not allow to be loaded into the memory unless you have the administrative privillege.
If you download or copy an exe file to your Win7 PC, sometimes you will get a prompt by the OS that this exe file is not trusted, do you want to continue. Windows 8 may have further tighten this security.