about computer slow down...

I post a message yesterday and I thank for all those who reply my post. Thanks for you suggestion.

Someone suggested disk defrag. I tried indeed. However, this application itself frozen in the middle of defrag.

The problem is obvious: 1) slow 2) crash or frozen if I open 4 or 5 applications. If I use defrag, this single application will froze the computer. (I checked thru task manager, seems this is the only application running)

Someone suggested me to rebuild from beginning. Please give me some specified instruction. Thanks...