Shafff, welcome to CF.
You can't legally download Windows 7 so you're doing something illegal which isn't supported by CF. So I won't help you install Windows 7, but I can guide you through getting Vista back, as long as you/her mom has the original Windows Vista CD/DVD that came with the Computer. You're most likely going to be spending some time with this machine now as there's a lot you have to do.
Insert the Vista DVD and boot from it to reinstall Windows.
Select the C: Drive to install Windows on.
partition OR format the drive if you want to retain the pictires on there. Just select the drive (in setup it will be the drive with the largest amount of space on it) and select Continue.
You will get a prompt telling you that there is already a Windows installation on the disk and that installation will be moved to Windows.old
Continue the Vista setup to completion
Now that Windows Vista is up and running, you'll need to restore the settings originally on the PC. This is where a lot of your time is going to go, making it just like dear ol mom had it previously.
To recover the files, open a Windows explorer and go to My Computer > C: > Windows.old > Users > [Moms old username]
Open another Windows Explorer to My Computer > C: > Users > [Moms new username]
Copy the contents from C:\Windows.old\Users\[username] to C:\Users\[username]. This includes the My Documents folder, Pictures, Video's, everything.
Now you have to go through and reinstall all the programs that mom once had (the fun part
You can browse through the Windows.old folder to see if there's anything else that needs to be retained or moved to a more accessible folder for mom. Most likely though, if she's the average computer user, all of her documents were in the folders that you already copied. I would say that you're safe to completely delete the Windows.old folder, but I would keep it just in case.