Lagging explorer (not internet explorer)

Hi.

For a couple of days now I have noticed that when I try to open a folder (or any directory in C:, for instance "My Computer", "My Documents", "My Pictures" or any subfolder out of these) it lags.
Specifically, the window opens without lagging but for a few seconds it is blank. Nothing appears except for the top bar with the minimise, maximise and close buttons. Everything else is white, as if it takes it a few seconds to load the contents.
It also lags when I try to close a folder window. If I press the X (close button) it wont close immediately but after one or two seconds.

Also this lagging becomes longer, the longer my pc is working. For instance if I open and close a folder when I have just opened my pc it will lag for just one second or less, but if I do the same after a few hours the lagging gets bigger.

This thing doesnt happen with programs. Every program opens and closes without lagging, so thats why I assumed the problem is in explorer.exe.

I didnt do anything radical to my pc lately: I didnt install any major program (just updated divx and had a recent Microsoft Update) or something that would make me suspicious that thats it.

So any idea what might be causing it?