It's about a 5 yr old laptop, so could be several things. Does it get hot? Check and clean the vents and fans (will need to open the laptop up to check fans). Try running it with a fan blowing on the laptop, see if that helps.
If it does not get too hot, you can try a new Windows setup to rule out any issues with Windows or other program causing the lock up, especially if you never installed a clean Windows setup since you got the thing. You'd want to backup your files/settings and use the restore disks or partition that came with the system. It will wipe the drive and restore Windows to how it was new.