It could be due to your CPU getting too hot or possibly your power supply packing up. Have you noticed your fans more lately ?
Might be worth removing the the side panel and hoovering any dust especially around the heatsink and fans and case vents.
There is apparently an issue with AOL so if you use AOL this might be relevant
The other possibility is you have removed a program without uninstalling it properly. It maybe still trying to load on startup.
Go to START RUN then type MSCONFIG select OK then select the START UP folder
Check through the entries of programs trying to stert up when you boot.
If you find anything that you have recently removed then untick the box. Likewise anything you do not need to start up when you boot up. That will be stuff like Office Winzip Adobe etc etc
Select APPLY then OK
The next time you boot the PC you will get a little message just tick the box 'don't remind me again' and select 'do not restart now'
Its also imortant to turn off Windows automatic updates until you get the PC stable again.
START CONTROL PANEL SYSTEM AUTOMATIC UPDATES select TURN OFF AUTO UPDATES