PC help! Anti-adware/spyware solutions.

I recently bought my wife a laptop and installed AVG anti-virus and Windows Defender (it’s running Windows XP). She uses Firefox. All was well for a couple weeks but in the last week she gets loads of extra Mozilla windows (not tabs) with all manner of ads (bingo, acia berry, adult friendfinder, online poker etc.) and the speed has slowed dramatically, not just her browsing speed but general computing speed, takes ages to open applications and stuff.

What can I do? We don’t have these issues with our home desktop.

Any suggestions? Any additional protection software I could install?