Is there a CD in your CD-Rom drive? Windows Explorer is notorious for freezing if it has trouble reading a CD (and Explorer reads the CD every time you open My Computer). I know some people have a habit of leaving a CD in the drive all the time. Maybe it's direty, maybe the lens is dirty, maybe the drive is failing.
When did this problem start occuring and did it co-incide with the installation of anything (hardware, driver, software)?
As Montag suggested, pull all your USB devices. It could be a misbehaved USB drive or card reader (be aware some printers now have card readers as well). I had a USB card reader once that would cause Explorer lockups - even with no card in the reader. It was having a conflict with another USB device.
Try using system restore to go back to a save point ONE WEEK BEFORE the problem started occuring (it's important, due to the way system restore works, to go back more than a day or two).
Could be the HDD is failing. Do you hear any clicking or clunking noises when this freeze happens? Download the HDD manufacturer's diagnostic utility and run a full (Advanced) diagnostic on the drive to be sure.
Hope this helps...