Your post is not really clear.
1. When do you have this? I assume it's after starting Windows in normal mode, but before the welcome screen or login box appears. Or do you have autologon enabled?
2. You "tried" to reinstall Windows 7. Does that mean that you couldn't reinstall it, or that the install went fine, but didn't help to solved the issue?
3. Was it really a clean install, including a format of the c:-partition? Or just an update over the existing install?
4. Are you using video integrated on the motherboard or a dedicated video card?

Things to try, assuming it isn't a hardware issue:
- Linux working normally (boot from a Linux live disk) ?
- Safe Mode working normally ?
- A really clean reintall of Windows (if the previous one wasn't)

Kees