Dell inspiron 1525 wifi problems

Hello, I got a dell inspiron 1525 laptop for christmas last year, an the wifi worked fine up until March/April, then it completely stopped working, during that time we did change internet providers 3 times, it worked fine with the first 2 but hasnt worked properly since the 3rd one.

What happens is, the wifi connects to the wireless router then maybe half a minute later disconnects, no matter how many times i do it, it always disconnects. So to go on the internet i am having to connect the ethernet cable from the modem to the laptop

I don't think its anything to do with the internet provider or the modem because it connects fine for my brothers computer and my mums laptop

Its just my laptop that it doesn't connect to, So can someone help me out please because I seriously cannot stand this any more