You don't say if you have Vista or xp. Sometimes this can be malware or a virus. Do a scan with your anti virus software and then a spyware scan.If that doesn't resolve it then there are other things to try.
Try clicking the start orb and type in 'device manager'.Or type in 'control panel'. I sometimes do this to access control panel when explorer kept failing to respond. Not a cure but a work around until it's sorted.