That laptop was released back in 2008. The overheating is likely due to thermal paste that has dried up. That basically means the paste could have basically solidified and now has air pockets in it. That acts like a heat insulator rather than a heat conductor. The best advice I can give is to take apart the laptop and clean out the old thermal paste and apply new thermal paste.
The following link has steps to show you how to take the laptop apart to that you can replace the thermal paste. However, taking apart a laptop can be quite daunting for some people especially if they are not particularly "electronics savvy" enough to do repairs.
http/www.irisvista.com/tech/laptops/Toshiba-Satellite-L355D/laptop-disassembly-1.htm
Linux is probably what you want to install especially since it is free. There are some companies that sells different versions of Linux like RedHat, but what they are actually charging you is support service. If you have issues with RedHat Linux you call them and they will help you resolve the issue. I have been thinking about installing Linux on my old 2011 Lenovo IdeaPad Y470 laptop... just because... But I have not had the time to do so. I hve been looking at Linux Mint which is one of the more user friendly version of Linux to install and use. Since it is free you need to rely on the Linus Mint community to help with possible issues.