Right click on My Computer, left click on properties, left click the advanced tab, click the settings button in the "Startup and Recovery" area. UNcheck the box that says "Automatically Restart" This will allow you to get the info on what the blue screen actually says. Click ok all the way out.
Reboot in safe mode (hit f8 when you see the white bar across the bottom on startup)
Open My Computer, right click on your "Local Drive" (or whatever your hard drive is called) Left click on properties, left click the 'tools' tab. Under the "Error checking" area click the check now button. Hit start to pick up the most obvious errors, then go back and check both boxes, you'll get a warning that you need to reboot. Ok your way out and reboot. The chkdsk will run during startup.
You may have noticed the disk defragmenter there. It doesn't hurt to run it but you probably want to solve the blue screen.
If you have added any software or hardware recently suspect that. Otherwise you may have failing hardware. The blue screen error message should give us more to work with
Be nice, be logical, be ready to define your terms...