internet keeps disconecting

my friend recently gave me her old laptop as she just got a new but it keeps randomly disconnecting from the router unpredictably as some times it will last for 3 hours sometimes only 5 minutes.
i know its nothing to do with the router because the other family laptop can connect to it fine.
my friend suggested a usb wireless network adapter and i found one but I don't know if it will help and don't want to waste my money.

Try the easy way first
Right click on the network icon, bottom right.

Then troubleshoot.

ive already done that
thats were it says to get a wireless network adapter aswell