installing windows

If you insert a windows boot disk (Installer disk), you bypass the ubuntu bootloader, so it is impossible for it to "stop you".

You should check your BIOS, and make sure the boot sequence is in the correct order.

This is Ubuntu support, not Windows support. I suggest you post on a Windows forum as your issue is with Windows, not Ubuntu.

Thanks

If your goal is simply to completely eliminate the Ubuntu system, then you can copy off any of your documents in the Ubuntu system, and then use the GParted Partition Editor to completely remove the Ubuntu system's partitions. To do this, boot from the Ubuntu Install CD, select Try Ubuntu, and then run System > Administration > GParted Partion Editor. The default installation of Ubuntu creates one partition of type ext4 (or ext3, for earlier versions), and one partition of type linux-swap. If your Ubuntu system was installed on manually specified partitions, then there might be more than two partitions, but the partitions would probably be of types ext2, ext3, ext4, and/or linux-swap. If you remove those, your Ubuntu system should be gone completely. Partitions of type ntfs are probably associated with a Windows system (and partitions of type fat32 are probably associated with older Windows systems like Windows 98). If you're new to this, and you have external storage devices, it's best to disconnect them before doing this, so you don't accidentally destroy data on them instead of on your hard drive.

But that should really not be necessary. When you install Windows, the Windows installer will give you the opportunity to wipe the disk and use all of it for Windows. As actionparsnip says, if you have trouble with that, you can get help on a Windows support forum.