boot sector

Hi, I'm having a problem with this computer I built for the 3rd time it works fine here where I build it but when I bring it to the person I built it forit works fine then by the time I get home I get a call and it's her telling me that that computer says insert boot disk and hit any key ...

Could someone tell me what might be going on?
I run the computer here and play games on it go on the internet and have no problem. This time I went and bought a bran new WD hard drive and set it up from scratch with xp professional and used it for 2 days booting it over and over again and it still worked like a charm, (like all my computers always do) but hers. I took it to her house turned it on and it was fine. I set up her email and set up my space and I set up everything she needed and it worked fine. I rebooted it there and it was fine. By the time I got home she called me and said it froze and now it says the famous saying, (insert boot disk and hit any key).
I have been repairing computers for 15 years and I can not figure it out.
Could it possibly be that I am setting it up here and I am running charter for the internet and she is running comcast or att whatever it is. From all the schooling I have gone threw and learned I doubt it could have anything to do with it.

alot of times when i used to get those errors was when i tried restarting my computer with a non-bootable floppy disk. If a floppy disk is discovered in the drive during start up it stops the startup and tries to boot the floppy but if the floppy isn't bootable, u get that error "insert boot disk, press any key".

so make sure she isn't starting up her PC with a floppy disk inserted.