During a normal windows session, use CTRL+ALT+DEL and note the processes that are running. Look for any CPU usage rates above a few percentage points. Memory usage is also useful to know although you should have more than enough to avoid your current problems.
When you talk about defragging, are you referring to Norton Speed Disk? The position of your files on the hard disk does matter, particularly if you use them frequently. If the skipping occurs at regular intervals, this is less likely to be the problem.
Again I'll mention that the problem may well lie elsewhere, but you may as well explore this and see if it helps.
Sex is great but it's no substitute for the real thing.....