Security Alert, inside pop up

Hello,

My Dell laptop has been acting weird...my 19 year old brother in law was
on it for a week...browsed all sorts of porn sites and also downloaded a bunch of video players...every 6 minutes a pop up message will show...pop up verbatim: title bar"Windows Security Alert, inside pop up:
"Warning! Potential Spyware Operation!

"Your computer is making unauthorized copies of your system and Internet files. Run full scan now to prevent any unauthorised access to your files! Click YES to download spyware remover . . ."

options are YES and NO
...lol I have tried to scan...will stall/freeze all scans..i tried the yes option and it froze..the no option will close the message...takes an hour to open up a icon or file...cannot get on Internet ...how can I fix? Also this has wi-fi and has always been on it...brother in-law also said he had to use other peoples wi-fi that he didn't know...he forgot Internet card and used whatever popped up..please help...thanks everyone!!