Hawai

My other computer is a windows XP and on the bottom of the desktop on my window security alert it says that my computer is infected with a virus and anti-virus soft pops up and scans.. it just popped up on my windows vista laptop right now but i just closed it and it closed and i scanned my laptop with my windows defenser and it shows no virus? but on my windows XP it wont let me do anything and everytime i click somethin it wont let me go there or on the internet do i have a virus on my XP and Vista?