It's not memory. Its disk space. Your main partition doewsn't have the space to store the temp install files.
One thing occured to me. I have seen this error when I had a USB storage device connected. The disk on this device was full. Windows puts files in places it shouldn't at times. Once I disconnected the external disk and started the install again it worked.
You don't have enough space on your disk to install it. As the message says you need enough room to store temp files for the install on the main partition. Clean some space or install on a new disk.
More than likely the version you get will be OEM and tied to the computer you buy. You could try calling Microsoft to activate, but, I doubt it will work.
You can uninstall on the old server and then install on the new. You will have to call Microsoft to transfer the key. If the key is from an OEM install then it is tied to the server it was installed on. If it is a retail key then you can activate on another server as long as you call Microsoft to activate.
The best way to do it is to set file permissions so that only the users you choose can access them. You can do that a couple of ways. Since I am lazy I use windows explorer and set permissions that way.You can also create user roles with appropriate permissions.
When installing in dual boot you always have to install the older version OS first. You could use Windows 2008 to restore the boot record and create the dual boot for you.