try explorer in safe mode.
Also, what progs start when you boot -
START - RUN - type MSCONFIG - STARTUP tab - some of the proggies there you want to keep - but some you might find are malware. Disable antivirus in this, and other non-essential progs, then reboot and see if that fixes the problem.
If yes, then enable one of the progs that you just disabled to start and reboot, see if the problem stays away - keep doing this till you have narrowed down the suspect.
Even then, it might just be the order in which certain prog/apps are loaded at boot which may cause a prob.
Good luck
before anything else, that will give more of an idea on the prob.