Dell Latitude C840 multiple problems

I've had this computer for a few years and never had a problem with it until now. I use windows XP. It was fine a little over a week ago. When I go to watch videos on say youtube or netflix the video itself gets all choppy and skips but the sound is just fine. I opened up Realplayer and watched a video I had saved and it worked just fine. The webcam I just bought for it works just fine in the smaller resolution. When the video in the browser messes up everything loads slower and sometimes will say the page isn't responding and ether to wait or to kill it (I use Chrome) and every now and then it will say a plug in has crashed, which now is usually flash player. I switched from AVG to Windows Security Essentials and found 2 trojans and removed them. I went through my C drive program files and started deleting files I thought I didn't need (some I wasn't sure what they were) and when I went to disk clean up it shut down on me and when I restarted it it had a blue error screen saying I was trying to delete something I needed. I went back and only deleted the things that I knew I didn't need (like uninstalled games). Now sometimes the computer will shut down randomly on its own without warning or error codes. One fan quit working a while ago but will decide to work when it wants to and both fans are dusty. I removed the keyboard to get to the CPU heat sink to clean it, but haven't turned it back on yet. I would get really hot depending on what I was doing but it never shut down on its own before. I've also removed a lot of things to free up memory. I have 37 GBs and only 11.5 GBs left. I downloaded Ccleaner and found out I have a lot of missing DLL files from uninstalled games or programs such as Photoshop CS2 and Elements 3, Silverlight (I think), Nancy Drew, and a few others. I'm afraid to delete anything just in case I need them. I'm not too sure what DLL files really are or what I need. I'm not the one who put my laptop together and I'm not that computer savy. In need of help really bad. I've had it up to here with this thing and am ready to wipe the hard drive but I'm not sure if it will do any good now. My computer will work fine for a little bit but then it will start screwing up. I can use desktop things just fine. Right now it seems to only be the browser.