Edit: with screenshots for each step... looked like noveltylamp beat my to the response, tho
Step 1: Use the Computer Management console to bring up Disk Management. There, select the drive that you want to have mounted. In the lower pane, right-click on the drive that you want to mount to bring up the proper menu option, as shown in the screenshot. (note: for hard drives, you can't do this on the drive itself--you must right-click on a partition that you wish to mount, as the screenshot shows... for other drives, right-click on the drive itself)
Step 2: You will see the list of current accessors for the drive. For each drive/partition, you may assign it only one (or zero) drive letters, and an unlimited amount of mounting folders. Use remove to remove the drive letter or a mounting folder from the list of accessors, use add to add a drive letter (if you currently have zero) or a mounting folder, and use change to change an existing drive letter.
Step 3: Click add and select an empty NTFS folder.
This is actually quite useful, because I have a few partitions that I've set up for data storage, but I don't want to have drive letter clutter, so I removed their drive letter assignments and instead mounted them as folders on the C drive. Great thing is, you can set up multiple folders for a drive/partition...
This is also especially useful on a LAN. I have two computers networked together, and I want each computer to be able to access the others' drives. Instead of putting each drive on share (default administrative shares like C$ or D$ are available only for hard drive partitions, not other drives), I just have every drive/partition mounted on my C drive so that I can access my drives by just going to \ComputerName\C$\Mount\Drive without having to individually share each drive. Pretty nice to be reading from my desktop's DVD drive with my laptop.