I have partitioned my hard drive and installed both Windows vista and ubuntu. I have not been using the ubuntu for a while and have since forgotten my password... So I was hoping to remove this ubuntu and re- install it,so how do I uninstall it?

You CAN'T uninstall an Operating system.

If you have finished using it then simply format the partition it is on.

If you installed Windows first and Linux second, then you can use this method.

Boot into Windows Recovery Console via your XP CD and run fixmbr to clear the master boot record (this is where Lilo or Grub is kept). Windows doesn't boot from the master boot record, it simply boots to the first active partition and loads boot.ini.

Once you do this and boot straight into Windows, simply right click My Computer, select Manage, go to Disk Management, and delete the Linux partitions. Then use those partititions for whatever you want or merge them back into your Windows partition with Acronis Disk Director or Partition Magic.

Why uninstall it if you are going to reinstall?

The ubuntu installer comes with a partition manager, you can simply remove all partitions (not the windows one) during the installation and let the installation use all unused space on your hard disk.