Defragging your PC can improve performance but only in some circumstances. For instance you need access to a LOT of small files quickly. Otherwise you'll notice next to no difference at all.
Also as for your Desktop problem, if you are using Windows 7, you can't save anything there unless User Account Control is set to its lowest settings. By default, you can't save anything to your root C: drive or any system folders (including desktop).
UAC in Windows 7 is actually pretty good. It stops you putting things where you shouldnt and generally keeps your PC organised. It also stops programs from executing malicious code (Viruses) and seeks your permission before changing any settings.
Yes its true that some PC manufacturers create D drive. This is usually a recovery partition. If your PC goes tits up, you can restore to the factory settings using a CD. The CD reads from the D drive to restore your PC back to the way it was when you bought it.
Having a D parition on the same hard disk as the C parition is pretty pointless if you are seeking to secure your files from annihilation. If your C parition dies, it's likely that so will your D partition since its on the same physical disk. Also, you would have to set the 'user' folders to an area on the D drive for it it make any sense. ie, point My Documents to a folder on D.
Special folders such as My Docs, Music, Pictures etc can be set to different physical locations by right clicking on them and changing their locations. So you can move it to a larger disk if you want to.
So in answer to your question, if you are using Windows 7, reduce the User Account Control settings if you want to write to the desktop (via Control Panel > User Accounts). But best practice is not to write to the desktop at all, as by your own admission, you're not that good with computers.