Any idea why my laptop won't boot up properly?

I’m not sure what details I will need to include here, but I’ll try my best. My laptop is a four-year-old Gateway and I’m currently running Windows Vista. It’s pretty much been through hell and back with viruses, wiped clean more than once, and I am keen on replacing it with a superior machine but not until the end of the school year.

Right now, I am running it in Safe Mode until I can hop-over onto my mum’s PC. Lately, I have been unable to properly load the laptop through my regular log-in. When I turn the computer on, I can get to the log-in screen without problem, sign-in, and see my desktop for no more than a minute. Then the screen dims to the point where I cannot see more than outlines, let alone figure out where my cursor is.

The first couple times this happened, I thought my laptop was overheating (which has happened – and is the reason why I have my fan on throughout the day) but after squinting and tilting the screen, I realized that I was still signed on and just couldn’t see anything.

I don’t know whether this is the battery, my laptop is just getting old, or if there is some worse internal problem than this not-so-tech-savvy mind can grasp. Sorry if this is too little to go by (I figure it’s probably difficult to give much help with these things without being able to tinker around with the machine itself) but I thought I should ask Fluther before considering taking it to anyone else.

I have a feeling this is going to be a long day.