Here's one of many articles on how to do that.
http://lifehacker.com/#!167525/change-windows-drive-letters
Hi there, please help.
I recently bought an external hard drive. It has worked very well for many months now.
When I used to plug it in the Windows\Sounds for Device Connected goes "****-Ding" as usual and "Ding-****" when disconnecting.
But suddenly after working so well I have a problem with my desktop computer. When I switch the device on it the PC goes "****-Ding" indicating the device is recognized, but now it fails to appear in my Windows list of drives in Explorer.
I see A: and C: and DVD D: , but the usual external drive G: never appears and is therefore unavailable for use. I now cannot do any backups and feel helpless and lost. I also backup to DVD but some of my backups (16 of them) are 70 Gig and DVD is useless for that size.
Please help me. Where do I look in Windows to rectify this problem? I can plug in my digital camera and "****-Ding" there it is. I recently plugged in a new GPS device, which uses F: and G: and it is available, but when I now plug in my external drive it does not appear as G: It does not appear at all !!
Do you think that the GPS G: is now stopping the external drive G: ?
Thank you.
ps. When I plug the device into my laptop and it works just fine. Drive G: pops up and I can use it on the laptop. The Desktop - NO, NO.
Thanks in advance.

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic