BleepingComputer.com: MSE Install Clobbers Booting?

This is an extension of another topic, where I was trying to figure out how to get McAfee on my cousin's daughter's Win7 laptop to work properly. It was popping up these two trojans about every few minutes as being found and quarantined:

TDSS.ac
Downloader-BMN.gen.e

Now, I lost patience with McAfee at some point, so I decided to uninstall it entirely, and installed Microsoft Security Essentials.

I set MSE up on its first Quick Scan, then told my cousin to just let it run. I left her house, and got a frantic call from her as soon as I got home. She watched it finish, agreed to let it clean two items it found (she doesn't remember what they were), and then it wouldn't reboot. Startup repair...that's what came up, but it wouldn't let her do that...it blanked out before she could, automatic or manually.

She now has a doorstop instead of a laptop for her daughter.

My next step may be going into Safe Mode (if it lets me), going back to a System Restore point before the MSE install, and figuring out what to do at that point (kill TDSS.ac?)...or we might have to reinstall Windows, but I have no idea if her daughter has the restore disks or original DVDs/CDs.

Any advice on this would be appreciated.

Some details are from memory...I'll pick up the laptop from my cousin tomorrow, so I'll have it with me for direct inspection.