USB flash disks not getting drive letter

Ok, so you only have a Disk0?

Try:

"Run "regedit" and select the "HKEY_LOCAL_MACHINE/SYSTEM/MountedDevices" folder. See all the "Dos Devices" listed? You'll have one entry for each physically mounted drive (HDD or Disc) plus you'll have one show up for each USB drive you attempt to insert. Double click each device listed and see if you see text saying something like "U.S.B.S.T.O.R...". Once you find that device, right click it and select "Rename". Now change the drive letter to something unused... like Q: or something. Close regedit, disconnect the USB device and reconnect again."

Have you Acronis TrueImage 2009 Home installed? The first version which seems to be still sold in boxes is known for causing such problems. There is an update availlable

Uwe

did you try updating the chipset driver?  if not, do that first.
 this can also help :
start>run>diskmgmt.msc      check if the usb devices show up in disk management, then run :   start>run>services.msc

Download devcon :  http://support.microsoft.com/kb/311272
then run :   devcon enable @usb\*

In the listing look for removable storage service.
If it is set to Auto, and not started, then start it : click the Start link in the top left...

Also, in the device manager, under Universal Serial Bus>right click each root hub, and select Properties>Power Management, and uncheck "Allow Windows to turn off this device to save power".

In c:\windows\inf, look for usbstor.inf    -  If it is not there, you can copy it from another PC..When you find it, right click it and select Install

Open the device manager, expand the USB section and then plug in the USB flash disk.  See if the device pops up anywhere.  If it does, uninstall the device and let Windows reload it.  If it does not pop up, then try selecting View ... Show Hidden Devices.  If it does not show up there, then you may need to add a System Variable in the Environment Variables table.  You want to add devmgr_show_nonpresent_devices   with a value of 1.  This will let you see all the devices configured using plug and play even though they are not plugged into the laptop right now.  They will show up grayed out.  Look for any USB flash disk devices and uninstall them.  

The theory here is that you have a damaged install of the flash devices and you need to remove it before Windows can reload it correctly.  If I'm wrong, at least you can clean up all the devices that no longer use from your plug and play installations.  :)

you used the same inf file for install ?  that is NOT what i asked, you must use another file

some more to try :
Remove old hardware devices :
go to Start>right click My computer>properties>device manager
in the view tab - click : "show hidden devices"
inspect  device manager for all hidden devices, and remove them

http://support.microsoft.com/kb/925196

OR :
Remove all USB drivers with:     http://www.ee-stuff.com/Expert/Upload/getFile.php?fid=6853      ---> Now reboot and test.
Delete the "UpperFilters" and "LowerFilters" registry values from:
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Class\{36FC9E60-C465-11CF-8056-444553540000}

you can also reset the usb :
rundll32.exe setupapi,InstallHinfSection DefaultInstall 132 C:\WINDOWS\inf\usbstor.inf
rundll32.exe setupapi,InstallHinfSection DefaultInstall 132 C:\WINDOWS\inf\usb.inf
rundll32.exe setupapi,InstallHinfSection DefaultInstall 132 C:\WINDOWS\inf\usbport.inf
rundll32.exe setupapi,InstallHinfSection DefaultInstall 132 C:\WINDOWS\inf\usbprint.inf

yes, i'm afraid so.