Comp slowdown when video file selected?

Hello all. I’ve been having a problem with my computer, where when I highlight a video file to play, the cpu usage of explorer.exe in windows task manager shoots way up, to around 80-90%. This only happens when I click on any kind of video file (.avi, .ram, etc.) Does anybody know why this is? I have a firewall set up, and I regularly update/scan for viruses and spyware. I am using Windows XP SP2, with 512 mb RAM and an Athlon XP 2600+. I use media player classic as my vid player, with appropriate codecs installed (i.e. divx, real play alternative, etc.) Oh yea, and I don’t necessarily have to open the file, just single click on it.