Once your in the recovery console from pressing "R' All you need to do is fix the master boot record "FIXMBR" as shown below... "Now at the prompt, type in fixmbr. Your damaged MBR will now be replaced with a new master boot record and your computer should now be able to boot properly. Note that you may also want to run the fixboot command to repair the boot sector with a new one." Please see link for details on this.. Here is a link that might be useful: fix-mbr-xp-vista

I would suggest putting free antivirus and malwarebytes on that pc before returning it if it does not have that and then if they end up messing it up again or getting infections it will be up to them to fix it, I think you have done your good deed by first giving them a working pc and fixing their mess up once! You have no idea what they are doing with the pc or where they are surfing or how well they understand and utilize security measures. That is up to them to learn and implement. There are lots of great free programs out there, give them a list and tell them to install them and run them. AVAST free antivirus malwarebytes free antimalware superantispyware free edition spywareblaster secunia personal inspector and most importantly all windows updates. You have definitely done a good deed.

my guess and I do mean guess is that it was a conincidence that it failed right after doing the updates, for DA it was not a drive failure but a corrupted/failed Operating system. It could be due to the fact the drive they provided you was not a new drive, but drives that are brand new sometimes fail. There is an old trick for trying to save info from a failed drive you might want to try it, you freeze the drive literally put it in the freezer,wrap it in paper towels put it in a sealed ziplock baggie, let it freeze well(a few hours or over night), stick it in the enclosure, or plug into the pc, and as fast as possible try to access it and grab your info, it will not work for long but many times it will work long enough to get the info off of it. Usually the window is about 15 mins. You can make it last longer if you put a frozen container or those blue ice packs on top of the drive when you take it out and connect it to the pc (protect it from moisture of course). Have a flash drive or some backup drive ready to go to get that info backed up immediately. You can repeat this a few times if you are lucky. Why freezing a hard drive might help on this site scroll way way down below all the blue text to read the info on how to and why freezing might work. does the drive spin at all? can you hear it spin up? is there any clicking or noise, does your pc recognize or see the drive? If it is spinning then freezing it is not the way to go because a spinning drive is not a dead drive. IF you want to try Spin Rite by Steve Gibson here is the site for it. SpinRite also this link talks of options to try including the freezer technique. Here is a link that might be useful: freezer technique and more

If you can set your bios to the selection to boot from the cd rom as zep has suggested then you can also use a linux live cd to access your windows section of your pc and at the very least retrieve your data so as not to lose it if you have to do a reformat and reinstall. Have you gone into the bios and made sure it is set to the correct boot order of cd rom first? Use Ubuntu Live CD to Backup Files from Your Dead Windows Computer Geek to Live: Rescue files with a boot CD that link provides directions and a picture of setting the bios boot sequence. It may be possible to repair the file also using the linux live cd but I am not one to tell you how to do that. In case you want to try using a linux live cd and don't have another pc to burn an iso cd, most book stores carry linux magazines and they all have free linux live cd's in them many with Ubuntu. You can try going to the Microsoft forum area at BBR and asking there too. Microsoft help the Linux forum there could be helpful All Things Unix