MacBook Air has a dark screen and works very slowly - what is happening?

Question

I have a MacBook Air since July '10, and until a few weeks ago, it seemed to be working fine.
A couple of weeks ago I wanted to update iTunes and GarageBand, but since I couldn't find my charger and I really needed to update my iTunes, I just let it update without a charger. It seemed to be fine and I think I either switched it of or left it on sleep mode overnight (I don't quite remember). The next day, I opened it normally and it worked very slowly, it kept freezing and that little colorful rotating ball appeared every time I tried to turn on a simple program, like Chrome or something. I restarted it numerous times (it froze so much that I couldn't even force end the programs) and left it turned off overnight, but nothing helped. A few days after that I backed my MacBook up using Time Machine and deleted stuff I don't need (I previously used over 80 GB out of the 100 GB originally available). I went down to about 30 GB of space taken up, and after that my Mac started to work fine, except for the occasional freezes. It still wasn't as fast as it was before (I couldn't open a few tabs at the same time in Chrome, or two programs at the same time, without it continually freezing), but it worked fine so I just assumed it was because of all the space taken up. The occasional freezes still annoyed me, though (plus I couldn't put pictures on it because I was scared that the freezes would occur again), so I thought I could reinstall Snow Leopard. It seemed to work fine, so I just left it there as it installed. A while after that, the backlighting switched off, as it usually does after a while. What worried me, though, is that even when I pressed F2 or moved the mouse or pressed something on the keyboard, the screen would not become light again. The light was extremely dim and I could barely make out what it said on the screen, but it was bright enough that I read a message saying that Snow Leopard failed to install, and asking whether I'd like to try again. I restarted the laptop and the screen was not only very dark (the white screen that normally appears was now dark grey), but it also took a really long time to turn on. A few minutes passed before the Apple logo appeared, and it took about 5 minutes (in comparison to the usual few seconds that it takes me to switch it on) to fully load. Naturally, it still worked extremely slowly and kept freezing when it turned on, in addition to the fact that the screen was dim.
Could someone please help me? I am completely not a Mac expert so I have no idea what the issue might be.

Answer 1

Usually signals memory problem.