Acer Aspire 5520 Series fan?

I got an acer laptop in June 2007, it was okay wasn't great. I bought it purely because it was pretty cheap considering the spec and i didn't know much about laptops or computers.

After a while it started cutting off while i was watching movies etc because it was overheating, i needed my laptop for work and college so i could never get it sent away and just had to deal with the fact it would cut off every couple of hours.

Then 2 months ago it started cutting off after being on for 20 - 30 minutes, so i couldn't get anything done, my flatmate told me it was most likely that the fan would need cleaned and she told me to hoover the back of it (Yes, i trusted that!) To me that seemed a pretty logical solution and it worked, until a few hours later when it cut off again. I then realised my fan had completely stopped working and my laptop now only stays on for about 5 minutes at a time then overheats and cuts out.

I don't have the money to get it fixed professionally so i was going to have a go at replacing the fan myself, as i read up online it's not too hard to do.

What kind of fan do i look to buy and how do i go about this?

Any help I would be sooo grateful!!!

xxxxxx