slow karmic system

[Joep L. Blom]

I have, actually from the beginning I installed Karmic problems which 
seem to be related to disk access.
System: AMD Phenom II (4-core) 4 GB mem; large HD.
Looking at gkrellm I see that I have 1,6 GB free and 6,7GB of 8 GB free 
swap space.
Nevrtheless the system constantly messes on the HD and I see the HD led 
practically continuously on. I have Vmware running with only 1 xp guest 
and of course when backup-c is active there is a lot of HD-traffic.
I cannot understand why for simple tasks like opening gedit and even now 
when I'm typing the system sometimes freezes for several seconds and 
after that I can type on.
After a reboot, the system is much quicker and "alerter" but after a few 
days (the system is now > 5 days up) it slows.
I've read somewhere that there is something like a "swappiness" factor 
that can be set but I have no idea what that means and I would very much 
understand what I do as I don't like to follow recipes without knowing 
the rationale.
Hope somebody can give me some clues.
Joep