Avoid third-party "ISO download" sites that ask for credit cards, offer "key generators," or provide EXE downloaders. These are almost always malware.
: Ensure your hardware meets the minimum requirements. You'll need a 64-bit processor, 512 MB of RAM (but 2 GB or more recommended), and at least 10 GB of available disk space. Windows Server 2008 R2 Standard Sp2 Download X64