My Inspiron 1520 will not boot up...the latest of several on-going issues.

I'm sorry ahead of time if this becomes lengthy, but I'm having several issues with my laptop right now. So thank you in advance for anyone who can help me.

I purchased my laptop in Dec. '07, and of course my warranty expired Dec. '08 right before all my problems started happening (how it goes, right?) I'm running Windows XP Professional, with AVG & Webroot Antivirus software, I use Firefox internet browser (now, I didn't until a few weeks ago). My laptop stays pretty much all the time on a Belkin cool pad I also purchased from Dell, and it seems to do the job of keeping it cool. I never have had an issue with it feeling too hot.

The first time I noticed a problem was when I removed the power cord to take the computer into another room, and when I unplugged the cord from the computer, it immediately froze up. This had never happened before, so I plugged the cord back in, and nothing happened. I had to manually turn it off, and back on. Of course, since I hadn't shut down correctly, I got a screen telling me that it hadn't and if I knew why, to just continue on but if not to boot in safe mode etc. Since I knew what I had done, I just let it continue to boot up, and it seemed fine. It did start to seem a little bit slower than normal and a bit lagging when on the internet, so I figured I just needed to do a good clean-up on it. I did the scans and clean-ups (temp files, history, cache, etc) that I know to do, but didn't seem to help and I also started to get these random, sporadic involuntary system reboots, where even if I was in the middle of doing something, it would reboot on it's own. It would also occasionally freeze up, where the only thing I could do is to manually turn it off and back on again. This went on for a week or so, and in that time, several Windows Automatic Updates were done and installed. I didn't know if that had had something to do with it, so I did some searching around online, and got some help via a tech. forum I've used before and they thought maybe I'd gotten some kind of virus or other malware issue, so they talked me through a deep clean of my system and other processes. It did seem to help somewhat with the slow Internet (We are on a wireless network - desktop w/router and my laptop and sometimes my mother's laptop - with satellite internet service). It could have just been coincidental b/c we do have to watch our bandwidth usage and sometimes that slows it down too. After I did the online tech help, this was going on 2 - 2.5 weeks, and I was still getting the random freeze-ups and reboots.

Which brings me to now, more and more freeze-ups were happening, even with the computer being idle and me not doing anything. One instance, I left the room to do something and came back maybe 10 minutes later, and it was frozen up, when it had just been fine. The more it started doing it, I started to figure out that as long as it stayed in one place, and I didn't move it, nudge it or pick it up to put it on my lap, it was okay, but if I ever did, it would almost immediately freeze up.  Also, the past few days, just booting it up initally has been a problem. I turn it off every night before I go to bed, and then the next day the first time I try to turn it on, nothing happens. The power light is lit, the hard drive activity light flashes, like it's going to do something, the CD drive flashes, the fan starts to turn. But nothing does, when I turn it off, I can hear a "click" sound that I feel like is the screen itself turning off? At first when it started this, I would repeatedly (probably not the best thing, but I didn't know) just keep pressing the power button and eventually it would show me the DELL screen. However, just seeing it, it would freeze up before the white bar could fully load to go to the next screen. I would manually turn it off, and back on, normally taking several times again, and then the DELL screen would finally load. If I could ever get past that screen, I could usually get to my log-in screen to put in my password. But several times, it would freeze up on that screen even before showing my desktop. So I would again manually turn it off, back on, until I got passed the DELL screen, and my log-in screen to it loading my start-up programs, but there again, sometimes it would freeze up in the loading process. (There aren't that many programs, just my AVG, Webroot, wireless network and the DELL programs that came with the computer.)

If I ever get past the loading of the start-up programs without a freeze up, and try to connect to the internet, most of the time within a few minutes, everything freezes up and the only option is turn it off manually. Also the past couple of days before it quit booting altogether, I did notice the right side of the screen, being a little bit more shaded than the left, a bit darker. As if the left side were in the sun, and the right side weren't. Not much difference, but enough to tell there's something not right.

During all of this, I did get several blue screen errors, but mostly they were after I'd done a manual power off/on. This past Monday night was the last time, I was able to connect at all somewhat successfully, Tuesday afternoon when I finally got it to boot up, I got a "please reseat the memory" screen. I'd never gotten that one before, so I thought maybe something had worked it's way loose. I unscrewed the two compartments on the bottom just to look, and nothing seemed loose, so I just reattached the covers and tried to turn it back on. It hasn't booted up successfully since.