Windows 7: Explorer "stalls" on some downloads

I have no idea how to track this problem down. Every couple of days after normal browsing and downloading, Chrome and also FireFox will "break" explorer at the beginning of any kind of a random download and the computer will stall and become pretty much unresponsive. It doesn't crash but stalls and the "aero" look of the task reverts back to default during the stall.

I don't get any errors in event viewer so I don't know what to even look for. I just have to reboot and then it's fine again. I can't find anything on Google so I have no idea why explorer.exe (I suppose) would just die randomly on random downloads. What should I look for here?

Thanks.