my notebook suddenly becomes slow and unresponsive but normal CPU usage and memory

The issue sounds to me as if you may have insufficient RAM. The least expensive, most effective upgrade one may do is to increase the RAM on their system.

I have had several customers who complained about their systems being fast at first then slowed down. Most were using the minimum amount of RAM recommended for the version of Windows 7 they were using. (1 GB for X86 & 2 GB for X64.)

I doubled the amount of RAM for these folks and their systems again became fast and responsive.

What happens is that with more programs installed that the HDD takes over for insufficient RAM, thereby slowing the system down quite a bit.