First check the PC Suite is the one recommended for your phone and your PC ie. Windows 2000, XP,Vista etc...
Next uninstall the PC Suite from your PC.
Don't install the PC Suite from your CD it's probably well out of date.
Please make sure you are using the newest version of PC Suite. You will find the latest version at http://www.sonyericsson.com.
Choose your region, support and your phone model.
Before installing the newest version please uninstall the current version of PC Suite and restart your computer two
If you can't remove the PC Suite by SETTINGS > CONTROL PANEL > ADD OR REMOVE PROGRAMS look at How to manually remove PC Suite