Inspiron 1545 problems

Last April I purchased an Inspiron 1545 with Vista. It has lived up to my expectations and genrally has been very good, with no problems until now.

A few days ago it overheated, not so much that it had to shut down, but I was alerted that I might have to backup my files. I did so, and I thought the problem would be over. However, the next time I logged on it ran very slowly and it took me around ten minutes to start up and log on. Then more problems came- I went on the Internet and after a few minutes it locked up and made it impossible for me even to shut down, so I had to hold the 'on' key to turn it off.

Now, everytime I log on this problem occurs and I cannot find any online solutions to it. Any ideas?