assuming your computer isn't 5 years old and knackered.....lol....
it literally could be anything. i don't think you should buy a new screen; it's something to do with the computer itself...maybe you're running too many programs at once and don't know it...if you press ctrl + ALT + Del a box comes up telling you how many processes the computer is having to deal with all the time....the lower the better....like don't have office and msn load up when u first boot the machine becuase it's unnecessary...you can really have agood dabble by going run>'msconfig' and unticking a load of boxes you know you wont need..though i'd invite sum1 confident round whilst you do that lol.........also programs like registry mechanic and just simply getting rid of files and applications you know wont be needed can help make it run quicker...
try not to run too many apps at the same time...like it you have word and media player open at the same time, then theyre sharing the computer's local resources, and if you have 20 windows open at one time then each will have to manage on a 20th of the computer's resources..
cheers