Cannot boot up, Webroot and Norton 360 on the same machine

Hi!
 
Earlier today, I installed Webroot on a Windows 7 partition of an iMac which had also had Norton 360. I turned off the laptop afterwards. When I try to boot up now, I only get to the password and when I enter it, the screen is completely black and it stays black. From time to time, I see the white mouse cursor--it flashes on the screen--and then it becomes black again. 
 
After a long while, it took me back to the screen to enter my password and then nothing but the blackness again. 
 
I was able to turn off the laptop so I booted up the Macintosh partition, just to make sure my hardware was OK, and that partition boots up fine (it has no virus checkers or internet security of any kind installed).
 
After several reboots, the Windows 7 partition still does NOT boot up and I don't have a restore disk. PLEASE HELP!
 
Here's some info:
 
- It's a Windows 7 partition. There aren't many software application installed and, except for the latest Chrome, I haven't installed anything new on it in a couple of years at least.
 
- I installed Chrome yesterday after using IE 11 in the past. I didn't allow the Webroot extension for Chrome. 
 
- Right after installing Webroot, I disabled most of Norton just in case the two didn't clash: turned off its firewall, turned off network drive scans, turned off all automatic scans including the boot scan. 
 
- Then I ran two Webroot scans successfully. As for the Settings, I only allowed IE Cookies, under Optimization, to be kept while everything else would be deleted. 
 
- Then I shut down the computer. I haven't been able to reboot anymore as I described above.
 
Please help! Thanks much
George