I have been running CS5 for over a year on Windows XP and today it won't launch. No messages-nothing. When I click the PS icon it turns light blue as if launching but then nothing.
I recommed resetting your preferences. Start Photoshop and IMMEDIATELY hold down Crtl + Alt + Shift keys. You should get a reset window.
A little tricky so keep trying. If you see the splash screen you are too late.
Do what Curt suggested. If it doesn't help, we need more info about the system to provide further assistance.