???hi i have vista running which is installed in c drive and i want to install ubuntu os(linux based free operating system)
link
http://www.ubuntu.com/products/whatisubuntu

but i want it to be installed in other drive like d.
But i don't know how to do that :-[
also if i somehow manage to install it how will i be able to boot it because if i start computer normally windows vista will start.Then how will i boot ubuntu at start up? ???

Nothing to worry about, just install it .. while installation , it will ask where you want to install then choose drive other than window.. it will show all partitions but with sd hda1 hda2 names.. so choose second drive, you can also guess your D drive by its size.

When you will install it.. after that when you will start computer, you will get screen asking for which OS you want to boot.. Vista ? or Linux so both can run on same machine but one at a time !
I have 4 OS installed on my machine

should i do clean installation ?
can i install it from windows ?

No, you can't install it from window.
You have to boot linux from ubuntu CD and install it on specified drive as mentioned earlier.

thanks for advice it will be a great help.