If you see BIOS (the white text on black screen) start tapping F8, this should allow you to get to the menu to select Safe Mode. Use Safe Mode to restore back to a earlier time and see if this resolves your issue. If you can't access Safe Mode or the computer actually POWERS OFF when your trying to start it then you have a more serious issue.
If no Safe Mode access: Windows itself is hosed AND / or potentially the Hard Drive is failing. Insert your Windows DVD and see if you can run repair, if you still get no success, then you will have to reinstall Windows, I would suggest making a DBAN CD first to wipe the drive clean, then do the Windows install then all your drivers, then applications. If during this the drive has errors (while wiping /installing) the HDD is bad and you then may need to consider two choices; 1) get a new HDD for around $129-$500 or a new PC for a i3Core at Walmart is $249 and a i5Core is $349 (both either desktop or laptop) as the price for parts as you can see tends to be more expensive then replacing the system.
2) If your computer just 'powers off' right away, it sounds like either it is overheating / shorting out. Have you opened it up and dusted it out with canned air? Cleaned any of the vents? If it is all clean and nice, then potentially it may be the PSU (power supply unit) is failing. Again the problem is a proper PSU would be over $99-$500, and might be more cost effective to buy a new PC instead.