You can buy a Windows XP if you are not sure what version she is running.
There are numerous versions of Windows XP (version options, packaging options etc) and you need to buy the one that exactly matches her version.
This problem could be a number of things - hardware problem, Windows problem, faulty hard disk, corrupt Windows system files etc etc.
Has anything been changed lately?
New hardware, new program installed, any other major change?
Did you get any error messages in the period (days before) leading up to it?