I tell people that not every problem with computers is due to a virus or worms or SPAM. They are caused by buggy software and people too.
She may need to have a XP CD on hand.
One thing you can try is booting off the XP CD. See if it leads you into REPAIRING the OS. Just have her read to you what is on screen. It should eventually give you an option to repair.
If the C drive is formatted in FAT32, have the computer boot and as soon as it starts to show the welcome screen or something, have her hit F8 as if there was no tomorrow. It will probably give a keyboard error beep. If so, then stop. Hitting F8 should take you into a menu where you can select COMMAND PROMPT. Go into it and type:
cd c:\windows ENTER
or (depending on how it was installed)
cd c:\winnt ENTER
Which ever one it takes you in, do:
dir system32 ENTER
If it shows you contents, then system32 exists and the problem could be anything and everything.
If this is the case, see STEP 1.
If you are looking at a reload, she will not loose her person info. Just programs installed. Now this will only work if the partition is in FAT32 (I am so against NTFS). Have someone, if she not have one already, create a 98 boot disk. Boot off of it. No formatting required. Just type:
ren window winbak ENTER
ren progra~1 probak ENTER
There, now you can install a fresh copy of Windows without formatting. Whatever the drive of the CDROM is holding the XP CD, type in place of the "x":
x:\i386\winnt ENTER
This will start the install. Just intall. All her personal files are still on the drive. If she saved any in \program files or \windows, they will be in the bak directories you renamed. Just move them over. I prefer to keep all personal docs and stuff, not i MY DOCUMENTS but I create a folder called "personal" right on C.
Not enough space for me to continue typing. I will add COMMENTS.