Why does my computer slow down and/or "freeze up" every night?

Every night, like clockwork, my computer gets reeeeeally slow around 3 AM EST. Most times it grinds to a halt, causing me to have to reboot manually (ctrl/alt/delete won’t even work). I’ve checked my anti-virus program, and it’s not scanning at that time. Something is running though… I can hear the computer “thinking”.

What could be causing this problem? What can I do to fix it? If it matters, I have a PC running Windows XP, with Firefox. Please keep in mind that I am a *technotard, so you will need to use small words in order for me to understand you. Thanks!

*Credit for technotard belongs to jmah.