Let's take it one at a time...

1) I am going to assume that your HD is properly located by your system since if it wasn't it would just say "Boot Disk Failure, Press Ctrl+Alt+Del to restart". This means most likely your boot settings are messed up, since you take no "ntldr missing" stuff. If that's the case, you need to boot from your windows CD. I trust that you use Windows XP as stated in your question. I guess you already know how to boot from your Windows CD. When the setup starts press R to access the recovery console. At the command prompt type FIXBOOT and press enter. Then type Y and it sould fix itself.

2) Does the reboot occur after a Blue-Screen crash? If it does, you most probably messed some drivers or registry entries. In that case use the command CHKDSK /F at the recovery consol, it should fix the problem.

3) There is a slight chance that your VGA or your CPU are getting overheated and they cause a reboot when Windows try to start. Try checking if your coolers work properly.

4) Even if all the above don't work and you need to format your HD, don't panic. Connect an external storage device on your PC and access the recovery console. There scroll through your HD, find the folders you want to save and use the following command.

xcopy [path of the folder you wanna save] [path you wanna save the folder to] /E /C /H /R /K

It should copy the whole structure of folders and subfolders including read-only and hidden files (if you want to exclude hidden files, remove /H).

Example: You wanna save c:\program files\photos folder and you wanna place it in your flash drive (f:) in folder f:\backup. Here's what you should type.
xcopy c:\program files\photos f:\backup /E /C /H /R /K