System Files Repeatedly Corrupt

For some reason, the system files in my Vista Home Premium system repeatedly corrupt. The symptoms vary -- typically PDF and other graphic intensive files cannot be saved or Windows Explorer crashes each time I try to use the right-click context menu. Each time the problem is cleared up by defragging my system files. The problem is driving me crazy because it repeats every few days. Anyone have an idea what is going on and how I can permanently correct this problem?