Is there any way to install Windows Server 2003/2008 on a HP MediaSmart Server EX495?
I take it there is no CD or DVD drive on that model?
Then you will either need to plug in a DVD drive into the motherboard (can't tell you how without seeing the internals), or use a USB DVD drive to boot the Windows installer DVD off.
Other than that there should be no problems installing 2k3 or 2k8,
You will need a copy of the Windows Server 2008 Disk (and a license), a USB drive and the following guide http://jesscoburn.com/archives/2007/10/15/installing-windows-2008-via-usb-thumbdrive/
Simply follow the steps in the guide and you will have a bootable USB drive that you can use to install Windows Server 2008 onto.
Alternatively as Matt has said you could just use an External USB DVD drive. It is simpler but you may not have one around.
The documentation at www.mediasmarthome.com/ says that SQL Server 2008 can be installed so from that I'm taking it as red that Standard 2008 server can be.
But I would advise to back up your system before doing anything so you can recover your data and servers functionality in event of a failure.
See page 6 for supported Operating Systems, supported means what HP will give support for on that hardware, other OS's may work also, just don't call HP for help when you have problems with an unsupported OS. I don't see any reason Server 2008 would not install and run just fine.