As stated, "Please re-install windows" is not a message I have seen before. Perhaps there's more to the message?
However, if your friend's computer does not boot up into Windows either in Normal Mode, or in the troubleshooting Safe mode, then it's not looking good.
If he has no installation CD or recovery partition, then the situation is looking even grimmer.
We know little about this machine except it is a Desktop, but no make or model, age, or any other details so all I can offer is;
1] If the computer is around 6 or 7 years old and came pre-installed with XP from the manufacturer, then they would have supplied the XP install CD and a drivers CD.
2] If the machine is between 5 and 2 years old, then no CDs were supplied, but instead a hidden recovery partition was created to return the system to factory conditions if needed.
3] If the system is less than 2 years old, then the first step recommended on unpacking and starting up is to create your own Recovery disks.
With most well known models, the manufacturer will supply replacement CDs for a small fee.
A friend claims the last time he tried to boot up his desktop with xp , he got a dialogue box saying "Please re-install windows " Says last he knows , kids were playing on computer . I know I need more info , but does this ring any bells out there besides he should get his kids some toys ? He also "lost" his os disc . Is there any way to get xp , or any windows os installed w/o buying it ? Thanks for any help !!!