Lately I have been getting lots of annoying ads popping up on my computer and its performance has slowed down significantly. What could be causing this problem?

Symptoms like this are most frequently caused by virus-like software called spyware. Spyware gets installed on computers through vulnerabilities in the operating system and web browser or is installed along with “free” software downloads such as games, screensavers, file sharing programs, etc. Instructions for removing spyware from Windows computers (the most frequently affected systems) are found HERE.