This might be worth a try.
For Windows 7 do the following.
Start > Run (or Windows Key + R), to open the "Run" dialogue box, in there type "diskmgmt.msc" and click ok, which should open up the Disk Manager. In Disk Manger check the bottom and see if it lists any CD-ROM drives, you might have to scroll down. See if you can spot the drive that it's showing in Explorer then rick-click on it and choose "Change Drive Letter", in the new box click on "Change..." then choose a new drive letter and click OK.
Then wait a bit, few seconds, and check if it now shows up in Explorer/My Computer with the drive letter you chose.
Edit....
If that doesn't help..
Start > Run and type "devmgmt.msc", which will open Device Manager, which basically lists hardware. In Device Manager expand the "DVD/CD-ROM drives" branch, then see if your disc drive is listed there, if it is right-click it and select "Uninstall". Then after its finished wait a few seconds, then from the top choose "Action > Scan for hardware changes", this will make it re-detect your ROM and re-install the drivers, once done it should also automatically assign a drive letter to it, if it hasn't try the first thing I mentioned one more time after this.
