That really is hateful when that happens! Id try a full spware scan & virus scan as Ramar said. Does it do this on all profiles or just yours?
If all else fails i find a reformat would work well, but that solves almost anything
edit - is your C drive very full, i found this was a problem for me shutting down, try goin to C:/ drive and then Tools and do the Disk check & defrag.