If you have any USB devices plugged in, remove them and restart.
If that doesn't work,
1. Boot from the Windows 7 install disk
2. After choosing your language, choose the "Repair your computer" option on the "install now" screen
3. Optional step: if you need special drivers to access your hard drives (SCSI, etc), follow the "load drivers" onscreen instructions to read them from an external source.
4. When prompted, choose the "System restore" option
5. Choose one of the previous restore points (I chose the second one down on mine just to be sure)
6. The system will roll back installs to that restore point; reboot only when prompted