you need a modem router to replace the BB modem you currently have (Are you on cable or standard ADSL over BT line as this changes which box you need).
Then you can either wifi both or have the main PC wired and the laptop using wifi. There is a certain amount of 'security' that needs to be applied but your friendly local techie will normally help out
There is a 'cheaper' way but it would mean the main PC would need to be on for teh lappy to get internet access and to be honest a router gives the main PC an extra layer of security thats worth having.
Your looking at ~£50 all in.