[SOLVED] selective drive letters not showing

Hi, i have an odd issue I cannot fathom. When adding a drive (USB/virtual/etc.) certain drive letters will show and certain ones won't. For instance, H, I, J, M, N, O, R, S all do not show in the computer directory but all the rest will. the drives show in disk manager but will not show anywhere else so if I want to use them, I need to open them through disk manager.
My machine has no malware/virus/spyware infection, W7 x64 reinstalled recently, BIOS up to date, all drivers up to date, no problems showing in device manager, view is set to show all drives. Have done much googling on this but to no avail.
I'd really prefer not to reinstall W7 or restore from a backup and was wondering if anyone else has this issue and knows of a fix or reg setting that would sort this out.
I've worked in IT for many years and never seen this so I'm not a newbie but it's just such an odd one i can't figure it out. I've checked in the registry and nothing is amiss, it just started happening out of the blue. Any help would be greatly appreciated.
Attached is a screenshot of current drive layout and lettering. In disk mangement I can change the letters no problem and they are all accepted but the drive just won't show with specific letters.

What happens in disk manager (diskmgmt.msc) when you right-click on the unassigned disk? Isn't there an option to assign them a drive letter?

If you have mapped your network drives and then installed new Removable Storage devices, the drive that relates to this device will not, by default, appear in Windows Explorer
Same problem, if you add a physical hard drive or CD ROM to the system after "mapping" a network drive
When a new removable device is added, the Manager cannot see the mapped network drive and assigns the next available drive letter to the removable device, and so assigns a different drive letter for the Removable Device
Try, under Disk Management, in the list of drives in the right pane, right-clicking the Removable Device(s) and then click Change Drive Letter and Paths.
Click Change, and specify a drive letter for the Removable Device, choosing one that is NOT assigned to the mapped network drives.
5. Click OK, and then click OK again.

Hi Guys, thanks for the response. All letters are available in disk manager and I can select or change any drive and it's letter, it's just that they won't show in explorer and it is only the letters I have listed. As far as the mapped network drives go, I have removed these and tried reassigning letters without them and still get the same thing. In disk manager the right click option gives me all available letters and allows me to assign any letter to any drive without a problem, it just won't show in explorer.

When you say they aren't showing, it's not just that you turned them off in windows explorer is it?

There are some tools that allow you to block certain drive letters, for instance with group policy editor, any chance you have done something like that?

Thanks Trainable, been down all these roads mate, I'm MS certified in W7 adminstration so I think I've covered all the bases, that's why it bugs me so......
I'll upload some pics of what happens.

The .doc in this file illustrates the problem for all aforementioned letters. I've just used 'H' in the pics.

I still think it's something in your registry set to hide certain letters and they just need turned back on. I use Tune-up Utilities and it makes it quite easy to hide drives but all Tune-Up does is tweak a registry setting somewhere.

Funily enough, I was running TU Utilities 2012. Uninstalled it, removed all traces of it from the reigstry, still same problem seen. Going to go for the backup restore from a specific time before I think it went wrong, seems only viable option at this point.....

F**kin computers, hate them man, f**ckin hate them!!...........mac's included...........