Computer not starting properly.

So here's the whole story:

I ran my computer (Dell XPS 420) perfectly last night. I shut it down for the night, wrote an exam this morning then came home hoping to enjoy some gaming time now that I"m on summer holidays.
I started my computer, it started normally.
I heard the new iPod 4.0 software came out so I downloaded and installed the new 9.2 version of iTunes from the apple website. I started downloading the iPod update.
While it was downloading, I decided that I would play some World of Warcraft.
While playing WoW, my computer froze up completely. Only sound was coming through. After waiting a while, I shut down using the power button on the tower. I tried restarting my computer and it started normally until it loaded the desktop and it froze. So I restarted my computer and booted from my Windows 7 disk. I ran a start-up repair, it found no problems and started the computer normally.
My computer ran fine for another hour or so. I didn't download the iPod update this time. This time i just played world of warcraft and talked to a friend on Skype.
My computer froze up again so I restarted using the power button.
This time the computer took about a half hour to reach the "Starting Windows" screen. Then it stopped, although the windows logo was still moving (it has that light beam animation).
After waiting about an hour, I powered down and tried booting from the disk.
The disk loaded and got to the "Starting Windows" screen and just stopped there and did not even load off the disk.
I powered down and let my computer sit and cool down (it's out in the open too so it has lots of air) for about 2 hours. I tried restarting using both of the above methods again with the same result.
I have some important work (not WoW) to do on the computer before Thursday so I need a fix for this.

Any help would be appreciated. I'm pretty good with computers so I'm up for most fixes.