Computer partitions, daul boot vista/ubuntu. uninstall ubuntu and install ubuntu server in its place?

Are you sure you installed Ubuntu into only one partition? If so, then boot from a live cd and -- WITHOUT MOUNTING ANYTHING -- open a terminal and do a sudo fdisk -l which should list all the partitions. Then choose what format you want to do it in and do a mke2fs or if you want ext3 mke2fs -j with the name of the partitions -- should be something like /dev/sda6 or /dev/hda6. That will nuke it. Thoroughly. It will be unrecoverable. NOW click on install (I assume that the live CD you are using is the Ubuntu server edition but I've never used it) and tell it to install to that partition -- and yes to use grub. Oh, you had better save your menu.lst to memory and rewrite it because you will have to reinstall grub or whatever your bootloader is to boot again.