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You can just boot into Vista and then use its disk partition manager to delete the partition of Ubuntu 8.04

Start – right-click ‘computer’ – manage. In ‘Disk Management’, you’ll see your hard-drives below (disk 0, disk1, etc.). Right-click on Ubuntu 8.04 partition and choose delete logical drive or some like that (make sure you know the exact partition to delete otherwise the result could be devastating)

BE AWARE!

Although the method that “minh” & “Mike H” has suggested is the easiest, but it may lead you to serious troubles in case you have GRUB as your boot loader.
Upon deleting the partition, although done from within Windows, the change will be automatically reflected in UBUNTU when you reboot since GRUB will, as usual, be reading the /boot/grub/menu.lst file to mount your root & other linux partitions. But this file is now outdated.

For example, assuming, originally your root partition was /dev/hda5, upon the deletion of a partition, the /hda5 will actually be /hda4. Since GRUB was not officially informed about the changes you made, GRUB will continue to think that /dev/hda5 is still the root partition when it has actually become the /dev/hda4 partition.

To overcome this serious problem, make sure that, after you had deleted the undesired partition from within Windows, you CREATE a new partition in that exact same space that was allocated to the deleted partition.
You can create the partition by right-clicking on the free space that was created by the delete operation, & choose create new partition.

Better yet, if you do not delete the partition. Instead, right-click on it & choose FORMAT & not DELETE.
This way, you will erase all data on it & will be prompted to choose the new file system(either FAT or NTFS) for this partition.