As your computer is started (after you first turn it on with power button), keep hitting the key F8 until you get a menu, then select "Safe mode". This should allow you to log in and get your documents, pictures, etc off (onto a USB devive).
It sounds like your computer has Driver problems or a trojan/virus that is causing it to shut down on boot. If you can access safe-mode then you can use this mode to try and fix your computer by removing software you recently installed, scanning with your antivirus & anti-malware tools (like AdAware Free from Lavasoft.de -OR- Spybot from safer-networking.org ...do not try to run both as the programs dont like eachother and will crash your system).
If you can not get into safe-mode due it rebooting then the only way to save your photos, etc would be to unplug the HDD from inside of your computer (its the big metal box normally screwed to the front inside your PC... or has an access panel on laptops). You can then attach this HDD inside a USB-HDD enclosure to plug into another computer... or change the pins (jumpers) on back of it to "Slave" and plug it directly into another motherboard (WARNING: The power should be off when you do this... but it would be even better to have an experianced technician do it for you).
Unfortunetly, if you can not get into safe-mode and restoring to a previous point in time does not work for you - then your HDD will most likely need formatted (repartition to be on the safe side too) and reinstalled :os
ALWAYS consult a technician before attempting to reinstall though... especially if you need files retrieved (and make sure to ENFORCE to them that you need the files retrieved).
Sidenote:
Some boot problems can be fixed quite easily with the install CD. Heres a how-to for Windows XP:
1) Boot from the install CD and choose the Recovery Console
2) Once logged into the DOS console (black and white writing), type the following, replacing [ENTER] with hitting the enter key:
fixmbr [ENTER]
fixboot [ENTER]
chkdsk [ENTER]