Cannot detect Sony Xperia in Eclipse

Question

Download the correct driver for your phone, make sure your phone has enabled USB debugging and then connect it to proceed with the install.

Download drivers from: http://developer.sonymobile.com/downloads/drivers/

Even though this question was posted long time ago, I didn't find 100% correct answer from previous replies. Most of replies are useful, but here is one final step for debugging apps on Sony Xperia devices.
My Sony Xperia T has not been visible in Eclipse even though I installed PC Companion and in mobile phone, I set developers options debugging mode!

Xperia has very strange option:
Settings > Xperia > USB Connectivity > USB connection mode
and select Mass storage mode (MSC).
Note that the USB cable must be unplugged to change this option. By default, Media transfer mode (MTP) was selected. If MTP is selected even little bug is shown in notification bar debugging is not possible!

These were my steps to solve the problem:

On SONY XPERIA Z

• Enable USB debugging (http://developer.android.com/tools/device.html#setting-up)
• Settings > Developer > USB-Debugging = Enabled
• Settings > Xperia > USB Connectivity > USB connection mode = MSC
• Settings > Xperia > USB Connectivity > Install software = Disabled (!!!)

On Windows7

• Google USB drivers were already installed for another device
• Download XPERIA drivers from: http://developer.sonymobile.com/downloads/drivers/
• Connect XPERIA via USB
• Device manager > Unknown device > Install driver from downloaded file

Don't forget to enable usb debugging in the phone setting.