This article shows how to install Windows server 2008 R2 on VirtualBox. Normally, Windows 2008 R2 server available for 64 Bit only, so obviously your computer processor and mother board shoud support 64 Bit architecture and VT.
Even though you are running 32 Bit Operating Systems, such as 32 Bit Windows 7 ,Vista and XP as host, still it’s possible to run 64 Bit operating systems as guest on X86 VirtualBox. To make sure your computer processor and motherboard support this feature read this post. The mentioned post is having tools to check AMD and Intel processors. Once result shows that your PC is 64 Bit is ready then you can proceed the installation of Windows server 2008 R2 on VirtualBox.