Hi there and welcome
When you say monitor, are you referring to an external monitor or the screen of the Laptop?
ok, it all points to an overheating problem, either too much dust or the fan not running correctly, but we should rule out any system corruption that can cause this
Click Start, All Programs, Accessories, right click on Command Prompt and select Run As Administrator.
A black window will open.
Type in it
press enter and when it has completed, type in
It will give a message "the type of the file system is NTFS cannot lock current drive.
chkdsk cannot run because the volume is in use by another process. Would you like to schedule this volume to be checked the next time the system restarts y/n" Say Y, press enter and restart the computer.
Please note, while this normally only takes 10 minutes to run, it can take at times number of hours if windows encounters many issues. If this is occurring it is critical to let this complete as it means you have corruption on the hard drive that it needs to fix. Even if it looks like the scan is not progressing, it is, please just let it run to completion. Windows will then start when it has.
Then tell me how it runs please.
If your Chipset supports it you can use Speedfan which you can download here
Though I still encourage you to do the steps I gave you