Don't try to start it, just boot into PW or the Windows 7 DVD/CD to clean the HD and reinstall.
Do you have data you need to retrieve from the HD, even though it may be infected?
Use this if so: http://www.techspot.com/vb/topic129757.html
If you want to try to rescue your install, it may be possible to install and run Avast boot scan and MS Security Essentials from the rescue disk OS.
Be sure to scan any data imported immediately using MS Security Essentials, which should be installed first after clean reinstall. Then scan it with Malwarebytes and Spybot to be sure. Finally scan the rescue disk and any Windows 7 Repair CD or Install DVD you use.