First of all: use the affected PC as little as possible! Every write access on your overwritten HDD could erase a little of your music library. If you are on this PC now reading this, shut it down! Boot this PC only from live CDs!
Secondly, what you will have to do in order to recover your data is not so easy ... so if you think you can't do it and you know a PC expert or something you should ask him/her for help. And don't panic! When you work this out slowly and carefully, there might be much to recover.
You should make an entire backup of your D: drive ASAP from which you could restore files later. When your Ubuntu installation erased the D: partition completely it may be best to backup the entire HDD.
To do that, you need a external or internal (not the one your D: partition was on) HDD which has enough free space and a Ubuntu Live CD. Everything you have to do is described here: http://www.howtogeek.com/howto/19446/make-a-drive-image-using-an-ubuntu-live-cd/
After that you can relax because you have now an image of your HDD to work with and no more files are lost from now on.
There are now two ways I know to rescue your music:
ddrescue (free, linux, command line, all file names are lost): http://www.gnu.org/software/ddrescue/manual/ddrescue_manual.html
If your D: drive happened to be an NTFS drive I strongly recommend that you use
Get Data Back for NTFS for Windows - it's not free to get but it worked like a charm for me, with folders and file names recovered): http://www.runtime.org/data-recovery-software.htm
(I'm not sure if NTFS Recovery works with DD Images but you could let it work with your overwritten partition too)
IF you're D: partition is totally overwritten by Ubuntu,I think you have to recreate the overwritten partition table in order to rescue your files. you can try that with Testdisk (it's also free and on the Knoppix recovery CD): http://www.cgsecurity.org/wiki/TestDisk_Step_By_Step