This is because since honeycomb Google have decided to use the MTP protocol to handle PC to Tablet file transfers as it allows them to obfuscate the internal file system on the device and fix one of the most glaring of the early problems with Android, namely that the space for applications and user data was fixed and one could be unable to install an application yet have plenty of space left to put your own personal data.
This means that you either need some special drivers which may be available from Microsoft or you manufacturer. You will also not have a drive letter for your device as it is no longer an actual drive as far as the computer can see.
Some manufacturers still allowed their devices to be used as USB mass storage (hard drive) devices, but it appears that Google has gotten its way.
You may need to install the Media Transfer Protocol Porting Kit to allow you to see and transfer files to the device.You also may need to get the latest version of Windows Media Player, but I don't know for sure on that one.
MTP is a big improvement over USB mass storage — for devices with lots of internal memory, a manufacturer no longer needs to come up with some hard partition between the USB mass storage and internal storage. Instead, they are all in one partition, with MTP providing access to the directory of media files that would normally be available through USB mass storage.This means there is no longer a need for apps on SD card for such devices, because what used to be the “internal SD card” is in the same partition as where applications are stored. The storage on your device can be used for either applications or media, depending on what you want to put on it. You aren’t stuck with how much space the manufacturer decided to leave for the two areas.