If you have not already, there are two ways to install the drivers. The simplest way to do it is to install the SE PC companion software, and the UPdate service (or just the update service) here:
Sony Ericsson - Support - Softwaredownloads - Detailed - Updateservice - Xperia™ X10 This will install the full compliment of USB drivers for X10's since there are several.
Follow all the instructions to install, reboot, and connect the phone holding the rightmost button down. This will make sure that the drivers are installed without answering the confirmation for either Repair or Update firmware. The software will install the USB drivers and putting the phone into flash mode confirms that the install is completely correct. If you just install the PC Companion software you should still be able to use the Sideloader to connect the phone to the PC.
The second way is to go to a 3P site like Davinciteam and download their SE USB driver package and install that. Again, you will have to connect the phone in SEMC flash mode and have windows acknowledge that the phone's software is installed.
BTW, NEITHER method works with XP SP3. XP SP2, good, Vista and Win 7 fine, but not XP SP3. You have to go into Safe Mode on XP SP3 to isntall and use the drivers in flash mode (but you only want to use ADB to install an app, so it shouldn't be a problem).