I am assuming that you don't move music files that iTunes knows about with Windows Explorer. If you have done this in the past. Let me know and I can help with steps to let iTunes know where the files are located now. Don't do anything else before we get that fixed.
If you have not moved music files that are also in iTunes, then you are ready to start. I always recommend a backup just in case something goes wrong. At least the iTunes folder and all subfolders should be backed up but if you want to proceed without a complete backup, that is your choice.
In Windows Explorer. create a folder on the other drive.
My suggestion is iTunes Media
long deep paths are trouble, but as long as it is the same length or shorter than the default (Typically C:\Users\<login>\Music\iTunes\iTunes Media), you are good.
In iTunes
Edit->Preferences
Go to the Advanced Tab
Note where the original path was (Typically C:\Users\<login>\Music\iTunes\iTunes Media)
Click the Change button.
Select the lowest folder you created (iTunes Media) so the path shows your new drive location
Click OK
Still in iTunes
Click File->Library->Organize Library
Check the Consolidate files box
Click OK
Wait for the process to finish.
Close iTunes.
In Windows Explorer navigate to one directory higher than the original path noted above.
Typically C:\Users\<login>\Music\iTunes
Rename the iTunes Media to OLD this will verify iTunes is not using the old files.
Open iTunes and play Music. Anything you select should play fine.
The original files are still on your C drive. C:\Users\<login>\Music\iTunes\OLD
Once you are convinced the transfer to the new drive worked. you can delete the OLD folder. Leave the iTunes folder where it is as that is where the library still resides. If you need that to be moved to the other drive too (not really necessary to move, as it doesn't take up a lot of space) then ping back and we can go through that too.