Wireless internet help

hey everyone, thanks in advance for your help, and sorry if this is in the wrong section
i know theres theres a million of these questions online, but i cant find the answer anywhere
my dads computer (a thinkpad, ya i know) used to connect just fine to our wireless internet and other connections, but now he cant connect to ours (he doesnt know if it works anywhere else).
the internet works completely fine when hes plugged into our router (linksys), but he would rather be able to movewith his computer, it is a laptop anyway.
he got a popup or notice this morning that said it was a firewall preventing him to connect...
the wireless works fine on my computer, ipod, cell phones, ect, its just his.
i just secure our internet too, and it stil works fine on my computer, would this matter?

thanks for you help, simplist solution would be the best, im not bad on a computer, but i dont want to mess with anything vital :)